Couple comments without judging your judgmental views:

-sperm doesn't stay in a female reproductive system (or digestive system for that matter) for years. It dies by the fifth day. And no, there isn't a bunch of dead sperm in a vagina for years. Even the epithelial cells (the ones your body would be touching) themselves don't stay there for years (they die off and are discarded frequently)
-If they used a condom, DNA technically never even touched her.
-If you can find a virgin girl who is okay with having sex with you and not being in any sort of relationship beyond sexual, go for it, but if you're not straightforward about it, you're an a-hole. If your reputation is ruined, that's just what happens.