I hope this response doesnt offend anyone!
Quite honestly i think it comes down to two things
the fact that women (generally) dont have the blatent 'fuck and go' mentality of most men.
I have been genuinely shocked at just how tv's and cd's advertise themselves as 'women' but as total and utter (im not sure what language i can use here) lets say v v v 'loose' and use extremely overtly sexual poses and want multiple men or other tv's at once.
I have been a woman for 50 years and i have never known any women to behave that way. Even most prostitutes only do it out of need, rather than the love of sex. I have been involved with trannies for just a year and half and have been totally astounded by the way men view sex. It is an area that genetic women are rare, so you get to see male behaviour at its most basic.
Basically men are just so different from women in terms of sex and how it is viewed.
Obviously this is a GENERAL view. There are women that love sex and have very casual sexual encounters, be that with men or other women, but on the whole the % difference is huge.
Thats my view anyway.