I can tell you a little about sex in esoteric Buddhism. First of all, its a very advanced practice. Its not something to be taken lightly. You can clean off a skinned knee and put a band-aide on it and then call yourself a "healer" in a sense, but that's not the same as performing neurosurgery. Its something like that. You got to remember these people study 20, 30 years before they get to the practices like that.

It's not a path I would go down, or am interested in going down. But its kind of like a fusion of male/female yin/yang forces as the other poster noted. Then the master manipulates the subtle energy in a certain way to become aligned with the cosmic cycles.

The medieval Western alchemists had some idea of this process and they encoded it in their alchemical seals and symbols...the idea of the "chemical wedding"...a lot of alchemy (turning lead to gold and so on) was elaborate secret metaphor for advanced workings with these high-level sexual energies.

See:
[link to www.levity.com]
or look into this stuff:
[link to en.wikipedia.org]

But in the west the science of this kind of psycho-sexual fusion was never as advanced and systematized as it was in places like Tibet and Nepal because it was driven underground and was Taboo. In Tibet you had monks working on this stuff for centuries, with the dedication of high-energy physics scientists.

Like I said, I think its all a bit above the heads of modern practitioners...you have to go through so much training before you can approach this level and just playing around with these forces is very dangerous...its OK to read about it I think, but not my cup of tea in terms of practice.