Is it a sin to have sex with promiscuous, unmarried women?

Question

According to Hindu scriptures, it is a sin for a man to have sex with most women who aren't his wives.

An ancient acharya says that it is permissible (not sinful) to have sex with your wife(s) and with women who have had sex with at least 5 different men.

There is also this verse from the Narada Smriti:

Nārada (12.78).—‘Intercourse is permitted with a wanton woman who belongs to another than a Brāhmaṇa-caste, or a prostitute, or a female slave, or a female not restrained by her master; if these women belong to a lower caste than oneself; but with a woman of a superior caste intercourse is forbidden.’

However, Medhatithi, an ancient Manusmriti commentator, doesn't agree with that viewpoint:

In the case of women who have not been wedded by any one, and live by prostitution, it is doubtful whether or not an expiation is necessary for having intercourse with them.

“What then is the right view on this point?”

The right view is that expiation is necessary.

Because the restriction (that one should he devoted to his own wife) has been directly enjoined, and expiation has been declared to be necessary in the case of one’s omitting to do what has been enjoined (11.41). ... The case of the ‘wanton’ woman has been already explained, and the prostitute also is an ‘unchaste woman.’

Are there verses in any scripture that state that this is permissible?

Answer 1

O Fair One! Thus I have described to you the characteristics, approved by S’âstras, of cow-killing (Gohattyâ) and murdering a Brâhmin (Brahmahattyâ). Now hear which women are (Agamyâs) not fit to be approached and those which are fit to be approached (Gamyâs). One’s own wife is fit to be approached (Gamyâ) and all other women are Agamyâs, so the Pundits, versed in the Vedas, declare. This is a general remark; now hear everything in particular. O Chaste One! The Brâhmin wives of S’ûdras or the S’ûdra wives of Brâhmanas are Atyâgamyâs (very unfit to be approached) and blameable both in the Vedas and in the society. A S’ûdra going to a Brâhmanî woman commits one hundred Brahmahattyâs; so a Brâhmana woman going to a S’ûdra goes to the Kumbhîpâka hell. As a S’ûdra should avoid a Brahmâni, so a Brâhmana should avoid a S’ûdra woman. A Brâhmana going to a S’ûdra woman is recognised a Brisalipati (one who has married an unmarried girl twelve years old in whom menstruation has commenced). So much so that that Brâhmana is considered an outcast and the vilest of the Chândâlas. The offerings of Pindas by him are considered as faeces and water offered by him is considered as urine. Nowhere whether in the Devaloka or in the Pitriloka, his offered Pindas and water are accepted.

The above verses are from the Devi Bhgavatam, Book 9, Chapter 34.

So, it is your misconception that one can cohabit with unmarried women without committing any sin.

The only women who are fit to cohabit with are the legally wedded wives who are called Gamya.

And, all others are Agamyas (unfit to cohabit with) for that person. It does not matter whether they are married or not.

And, some women among them are of course Ati-Agamayas (extremely unfit to cohabit with). One's own sisters, relatives etc fall into this category.

UPDATE:

Apasthambha also agrees with the Devi Bhagavatam and states:

It is a sin to engage in sexual intercourse with a woman who has been married before, or whom he has not married with the proper rites, or who belongs to a different class than he; 4and a son born from their union undoubtedly participates in their sin.

Apasthambha Dharma Sutras 2.13-3,4

Answer 2

When we have sex with more people, the body builds up karma which have to be cleansed before any real spiritual enhancements. So it is regarded as sin because it is bad for your progress because always the ultimate aim of this culture is moksha. If you are don't want any spiritual progress and life to be smooth, I don't think it will be a problem.