Freud.

http://en.wikipedia.org/wiki/Philosophy_of_sex#References_and_suggested_readings Looks like quite a few. Acquinas, Kant, even John Paul II Although, I'm sure the line between sex, and love is blurred in many of their writings.

I'm not sure but I doubt if most philosophers had a lot of sex, so, that could be a problem right there.

Me! Alla time! ;-)

It really depends on context - speaking strictly philosophically, sex per se is not a particularly interesting phenomenon, so there aren't a lot of philosophers who focus on it alone. For some reason, the most frequently linked area of philosophy to sex is ethics - but, to quote Peter Singer, "[S]ex raises no unique moral issues at all. Decisions about sex may involve considerations of honesty, concern for others, prudence, and so on, but there is nothing special about sex in this respect, for the same could be said of decisions about driving a car." Other than this, the other main field it could coincide with is the ever-present discussion of the foundations and value of happiness - and the high correlation of particularly pleasurable sensations with (potential) propagation of the species is a useful data point in evolutionary theories of the origins of happiness but nothing more, again providing little for philosophers to work with. If it's considered as an act *within a social framework* then, sure, there's a LOT of discussion to be done - questions of power relations, identity formation (and transformation), the legitimacy and power of social norms, etc, etc, etc, and a lot of modern philosophers, who focus on this (generally from, or at least with strong reference to, feminist perspectives) - but strictly speaking this isn't a question of the philosophy of sex as a concept in and of itself, but of the social constructs that shape (and are shaped by) it. When it all comes down to it, there's no *philosophical* content to fluid exchange.