Yeah its the acid in saliva that does the harm I think.
I don't know about soap, but I assume it would be a similar thing, there are so many chemicals in soap. I guess after you've done the bum on a pillow for a while and rested it won't hurt, I mean any sperm that you wash away with soap will only kill the ones on the outside...
lol there r loads of theorys tho isnt there like legs in air for 30mins but then sperm does wat it needs 2 in the first few seconds coz its put up there!!!! ppl get preg from being on top!!! xx