I am just curious for what religious reasons there might have been. In certain sects of Hinduism, women are asked to not enter the kitchen, eat and sleep separately from the rest of the family, not enter temples, etc. when they are menstruating. One obvious reason may have been sanitary reasons, but it is still followed in 2014 when it is no longer a reason. Are there any other, religious reasons for this?
Menstruation is considered to be "muci," or impure. For many brahminical families, temple worship and puja were their primary occupations. One cannot worship the Deity or perform rituals if one is unclean. The concern was that women would contaminate the offerings prepared.
Manu-smriti / Discourse V - Sources of Evil / Section IX - Other forms of Impurity
(Sanskrit text, Unicode transliteration and English translation by Ganganath Jha)
रात्रिभिर्मासतुल्याभिर्गर्भस्रावे विशुध्यति ।
रजस्युपरते साध्वी स्नानेन स्त्री रजस्वला ॥ ६५ ॥
rātribhirmāsatulyābhirgarbhasrāve viśudhyati |
rajasyuparate sādhvī snānena strī rajasvalā || 65 ||
In the case of miscarriage, the woman becomes pure in so many days as there have been months; and the woman in her courses becomes fit by bathing after the ceasing of the menstrual flow. — (65)
दिवाकीर्तिमुदक्यां च पतितं सूतिकां तथा ।
शवं तत्स्पृष्टिनं चैव स्पृष्ट्वा स्नानेन शुध्यति ॥ ८४ ॥
divākīrtimudakyāṃ ca patitaṃ sūtikāṃ tathā |
śavaṃ tatspṛṣṭinaṃ caiva spṛṣṭvā snānena śudhyati || 84 ||
After having touched the Cāndāla, the menstruating woman, the outcast, the woman in child-bed, the dead body, or toucher thereof — one becomes pure by bathing. — (84).
On a different note, women were allowed to fully rest during menstruation and were excused from all household duties. This led to quicker recoveries and happier women.
The practice of separating menstruating women from the household is based in the concept of Ritual Purity & Ritual Contamination in the Vedic tradition. Ritual Purity and its understanding is a major aspect of any religious tradition. At its heart, there is a belief that humans exist in a natural state of uncleanliness, and this 'uncleanness' is not just physical, but spiritual as well. Any emanation from the body is considered unclean. Association with the divine requires purification at all levels. For example, within Christianity, the act of Baptism cleanses the child and allows him to receive God's grace. The act of Confession is necessary to receive the Eucharist, the body of Christ. Wazu purification is necessary to offer namaz. So on and so forth.
Within Hinduism, Ritual Purity is understood to occur at many hierarchies. The Gods (and deities) exist at the highest level of purity. The Priests who tend to them must exist at a similar level of purity. For the Priest, this purity is achieved by the thrice daily recitation of the Sandhya Vandanam (ie, the Gayatri), achaman, and similar purifications. This allows the Priest to now associate with the deities. However, if he contacts anything (or anyone) 'unclean', he must repeat the process. Unclean acts include sex, sleep, eating, sneezing, vomiting, etc. Any emanation from the body is unclean. By the same logic hair & nail clippings are also considered unclean, once they're separated from your body. A menstruating woman is thus considered 'ritually unclean', due to active emanation from her body. Similarly a Priest with an oozing wound (or a runny nose) is ritually unclean. There are also levels of purity. Normally, leftovers are considered highly contaminating. The emanations, or leftovers of people higher up the purity scale are purifying for people lower down. The leftovers of the Gods is purifying for us humans. Tirtha, the bathwater of the deity, is purifying for us. Similarly, the leftovers of the Guru is revered by his followers. In traditional families women eat the leftovers of her husband. A mother can eat the leftovers of her children and not be contaminated.
Most Hindus are more familiar with the related concept of Sutak which describes a condition (temporary or continuous) where ritual purification is not possible, for any number of reasons. Most people have a great misunderstanding about what Sutak means. Contrary to popular belief, Sutak is not a state of EXTRA uncleanness, but rather a condition in which ritual purity has been reset. The Preta-kanda of the Garuda Purana also describes levels of Sutak, most of which are removed by bathing. These are understood to simply be conditions where the pre-existing ritual purity is lost, and can be regained by the regular methods. For example, upon the birth of a child, the father also has sutak, but in his case this is removed by bathing and the usual ablutions. The mother, however, has Sutak for 10 days, due to the continuing secretions from her body, which would nullify her attempts at ritual purification. This is no different from a person with a runny nose... or diarrhea.
In addition to what has been answered by @cheenbabes - To quote specifically
it is still followed in 2014 when it is no longer a reason. Are there any other, religious reasons for this?
In many sects within Hinduism, It is a command of God. Like in Swaminarayan sect, it is mentioned in the Shikshapatri Verse 174.
To add to @cheenbabes answer,
Here is why women were not allowed to perform any rituals:
Puja should be done only with Sattva dominant body and mind
During the Menstruation period, there is "Rajas" nature dominant in female body (high emotional activity).
Menstruation is considered as ashaucha (ritual impurity) that is why it is advised not to touch a menstruating woman.Some verses from scriptures which clearly say menstruation is a ritual impurity, and this impurity is temporary only. It helps woman to realise there impurity or sins.
Vasishta Dharma Sutra V. 4-5 “For month by month the menstrual excretion takes away her sins. A woman in her courses is impure during three (days and) nights.”
Baudhayana Dharma Sutra 18.104.22.168 “Women (possess) an unrivaled means of purification; they never become (entirely) foul. For month by month their temporary uncleanness removes their sins”
Purity is an important, Shaucha (ritual purity) is important, that is why it is advised not to touch a menstruating woman.
Manusmriti 10.63 “Abstention from injuring, truthfulness, abstention from unlawful appropriation, purity and control of the sense-organs,—this Manu has declared to be the sum and substance of duty for the four castes.”
My answer will take view from scriptures and not the today's pseudo science
Why Menstruation occurs ?
Once Lord Indra deva offended his spiritual master Brihaspati, and eventually declined in oppulence and peace. Suggested, he approached "Visvarupa" a demigod with 3 heads and son of demigod "tavsta". Ordered by Indra, he performed sacrifice to raise the pious credits of Indra, but with one mouth, the 3rd mouth started supporting the Asuras and when Indra noticed that he took out weapon and killed the Visvarupa. Enraged his father "tvasta" took revenge, which is another story. But due to killing of Brahmana, Indra did the greatest sin of Brahma hatya and now had to suffer. Eventually story goes in the way that he finds the way of getting rid of it by distributing the sin over 4 bearers and giving boons to them. Women were one among them. They took the 1/4 the share of Indra's sin and eventually started menstruating. Indra on return gave the boon that women will have more sexual powers than males and enjoy more sensual pleasures. Please don't take it otherwise, it again has many philosophical backgrounds, I better avoid here.
What is the state of women undergoing menstruation ?
Shastrik injunction and scientifically both, since its a combination of rotten blood and flesh , it makes the body impure and contaminated. Cooking food and serving someone not only involved feeding grains, its a tansfer of consciousness. Why ? have you ever wondered why a roti baked by mother satisfies so nicely but not by the dhaba walas ? because mother transfers the consciousness of love and care in the food cooked. Similarly when menstruating the consciousness remains impure too while thinking about the bodily conditions and hence women should avoid cooking food and other activities that involves serving family.
Why barred from temples and other religious activities ?
Again same reason, impure consciousness due to thinking about genitals, blood, bodily conditions etc. God is not material he is spiritual, Supreme Consciousness and so "A consciousness can be served by consciousness only"
Who should follow ?
Every women, its a myth that only brahmans should follow but its about every women's health and her family. Those who say its supersition are simply fools. Other than humans many other mammals menstruate and they don't take care of hygeine and bath etc. In this way humans are becoming more like animals these days. Die unto western philosophies, who degraded the society to animal consciousness.
What scientific basis you put for it ?
Scriptural clarification itself is sufficient, but out there there are many followers of the theory of evolution who demand scientific basis. There is, some have already been mentioned. During periods a lot of blood is lost may times. This makes them physically weak and emotionally drained as you already know that Progesterone and Oxytocin hormones levels have steep changes giving rise to drastic mood swifts. This mood actually we call connection to the consciousness. Due to blood loss they need proper rest and care.
Hope this gives answer to many questions. Please feel free to ask.