I think you completely misinterpreted the film.
I would go as far as saying he definitely didn't rape her -- it would conflict with who he was as a character and the story -- it was opaque simply for the effect of putting us in Simon's shoes/mental state. Gordo wasn't raped, Simon lied to make it seem so - this is mirrored by Gordo's modus operandi towards Simon in the third act. I don't think Edgerton was trying to make the audience feel like bullying was the acme of human degradation, at all. It was a simple case of, as by Gordo's mantra "an eye for an eye" - just like Simon's lie ruined his life, he wanted the same for him - whose demise could be argued as being self-inflicted through being a pathological liar.
It was a lose/lose situation for Simon - does he ask for a paternity test, knowing full well that if the baby turned out to be his, the circumstantial evidence could place doubt in his wife's mind as to who the guilty party might be [one behind the mask] - meaning whatever chance he previously had at saving his marriage would now be beyond rectifiable. Or does he live the rest of his life without knowing if he's the father or not?