Cuz you get water pushed up into your vagina, causing nasty infections.. Especially if its in a swimming pool or something, you don't know whats in that water! Which kinda sucks in my opinion cuz thats always been a fantasy of mine, to have sex in water of some kind... But as close as ive got is in the shower.. So that'll have to do I guess.

Also the water washes away the woman's natural lubrication, which makes sex more uncomfortable (at best) and painful (at worst) though there are special lubricants that stick with you in water ( they are silicone based, which should not be used with condoms so if you're practicing safe sex, then you shouldn't really have sex in water). But yeah esp in hot tubs there is alot of potential for all kinds of infections.

Yep I agree...I did actually have sex in the pool/shower yadayada...And it wasn't all that great as I expected...Now sex in the rain...Thats a whole different thing..

Well, the one I heard (and it is probably another old wives tale) is that you should have sex in the ocean, because as the waves moves in and out it can create a sort of vacuum effect and you end up being sort of suctioned together...I've never heard of this actually happening to anyone though!

Haha...Never heard of that one...But I guess a perk to that would be well..Not only the suction lol, but the fact it's salt water...Which would take care of the nasty infection..So there ya go ladies...Sex in the ocean

Other then infections it can't cause you any harm persay however the whole getting suctioned together thing .I.S. True!!!! The water pressure and the movement of sex can cause it to get "stuck"!