Before visiting Shabarimala every devotee has to take up vratham (penance) for 41 days and should stay away from alcohol, smoking, non-vegetarian food and all other tamasic things including sex, cutting body hair, shaving and even trimming the nails and should not sleep on bed. They are expected to bath twice in a day and visit the local temples regularly and only wear plain black or blue colored traditional clothing during this time.
A crude explanation is that a man who has practiced celibacy for 41 days will get distracted and loose his spiritual concentration around women devotees.
As far as the myth goes,
Another story related to Malikappurathamma goes like this: After
Mahishi was killed, a beautiful lady appeared from the body of
Mahishi. She requested Ayyappa to marry her and accept her as his soul
mate. But due to his mission and the state as a Yogi, Ayyappa refused
it. But due to the incessant beseech of the girl, the Lord gave her
the word that he will definitely marry her at that time when there
will be no Kanni Ayyappas in his abode at Sabarimala.
So, it's said that a fertile woman entering the shrine will 'enrage' Malikappurathamma.
Another motivation to discourage fertile women from entering the shrine is the celibacy of the presiding deity who's in the state of sitting called Arddhasana, Yogapadasana or Yogarudha Siddhasana. This status of sitting reveals the mood of asceticism, the Tapobhava.
Since women are considered to be the source of sexual energy, a nitya brahmachari deity has put certain restrictions on female devotees.
A more liberal and political correct view might be that it intended to ensure that women are not put through the difficulty of traversing the mountain under rigorous conditions and their comfort and safety too could be an area of concern on a pilgrimage like this. Women who could be bringing up a family. Remember, this was the case when there were no motorized vehicles and the forest around Shabarimala was notorious for the tigers and other wild elephants marauding humans.
It's silly to assume that Shabarimala Temple Trust is being misogynistic, because they are preventing 'sex and sensuality' from their premise and not women as kanya (before a woman becomes fertile) and post-menopause (after 60) woman are free to enter the temple and worship.
Recently I came to know that the group 'Indian Young Lawyers' Association' which filed a PIL against Travancore Devaswom Board (TDB) is headed by a muslim lawyer by the name 'Naushad Ahmed Khan' who is a resident of Delhi.
There's a conspiracy theory doing rounds on social media about hidden agenda and ulterior motives behind this PIL. Some even saying that if a muslim man wants to establish gender equality he should first do away with the regressive muslim personal law which treats muslim women differently from say Hindu women.