Well, it's really very simple. Lots of people think about crazy stuff, they just don't admit it... we do.
If you go to a filesharing program, and make some searches looking for some sex clips, odds are u gonna find a lot of weird stuff. A lot of threesomes, gangbangs, creampies.... If they are there, is because people have them. But then, they don't admit their own desires.
Another example, ask any man if he would like to have a threesome with his girlfriend/wife and another girl... he will say YES! Well, it's really the same thing for girls, they aren't made out of wood.
Then ask the same guy if he would like to see some bi-lesbo action during that threesome... he will say yes. Then why can't be the same the other way around. That doesn't mean people are gay, not even bi...
About the why this happens, there are a LOT of theories out there. I preffer to believe in the theory of the new hole. This one states that after banging the same hole day after day, it becomes a rutine and looses it's lust. Then if a new hole appears, it may not be a preety as the old one, but it's new and will work like hell. An extension of this theory states that after a lot of normal/clasical spear/hole action, the act itself becomes monotonous and you start craving for something else. Not that normal sex it's not good anymore, it still is good, good and necesary, but it's no longer the 'ultimate experience'. At this point, for that 'oh my god!' feeling you need to go beyond.
And... well I think I'm talking too much already without really knowing what r u looking info about. Let us know what troubles your mind... and maybe it should be a new thread (I think).