I don't think that the "pre-marital sex is sinful" is a myth so much as a belief and matter of opinion. I mean, a myth is something that is just factually untrue. Like, if you swallow semen, you can become pregnant.
Personally, I don't care if people feel that way about pre-marital sex. It's their life, their beliefs, and they can let it guide their behavior if that makes them happy. As long as they don't try to impose it on other people, I don't care.