They are horny before their period, they are horny during their period and they are horny after their period. PMS is actually the time when it dawns on them that they went to the trouble of producing an egg, they failed to fertilise it and now they have to go and make another frigging egg. Life would have been so much simpler if they made the egg, fertilised it when ever they had time and didn't produce another egg until they felt like it. All this business of making the egg, getting rid of the egg and making another egg just because the moon is full again is just so tiresome. So, when women are horny, they have sex.
Women pick who they have sex with.
That's if they feel like using a man at the time. They are perfectly well equipped to enjoy sex by themselves or with each other. The only reason men enter the equation is procreation. I hear you ask. What you need are more ewes to produce more ewes.