I understand what you're saying, but do you really think he was thinking "I have random people's DNA all over my body if I pick her up/lie on top of her maybe some will rub off!"? Forensics were not as developed, and I doubt people were thinking of things like touch DNA at the time. They weren't criminal masterminds. Even if they watched a lot of crime shows, their alleged scenario didn't necessitate contamination to make it work, unless they were going to go through the trouble of stealing someone else's semen or other incriminating material. They'd certainly know enough to clean off anything incriminating, but "valuable evidence rubbing off onto his body"? Like what? If it was evidence of someone else, it's unlikely it would all rub onto his body, plus that would mean he wasn't responsible. If it was evidence of blood or semen or fibers that belonged to him, it would still be unlikely to all suddenly attach to him and remove him from suspicion - he would have washed that off earlier if he thought it was still present. I just don't see what role contamination plays in this case - they probably had no idea that police can examine little fibers and skin cells to this degree, particularly when he's carrying her on the outside of her clothes where his hair or something would be expected.
And I don't understand how DNA from someone at the party can easily get under her clothes. Possible, but doesn't seem particularly likely. Not saying I don't believe the Ramseys were involved, and they weren't cleared, but I don't see how DNA on her underclothes can be dismissed so easily.