Oh wow, I can't pick just one...
Ok, so, "size matters" to some people, to each their own...but assumption that it matters to anyone and everyone, makes someone more of a man, and is always a big factor in the pleasure of vagina-owners...SUPER ANNOYING.
I don't think most people believe that pre-marital sex is sinful. That's less of a widespread myth and more a disappearing attitude.
"Women don't like porn" is annoying (most porn isn't made with women in mind...) but it's part of a greater myth that women don't like sex that much/experience desire, when in reality there are often few ways to express that desire without being degraded for it.
A myth I would like to add is that there is one way to be "good at sex."