He slept with the henchwoman without killing her, so in movie-verse (as well as SV-verse) Kryptonians can safely have sex with humans.

Oh, now that you mention it, I have a very vague memory of that. (Superman III is the one with Richard Pryor, right?) Weird, though, because it makes Superman II just bizarre.

My personal favorite solution to love triangles is generally threesomes, but I suppose that's unlikely to happen onscreen.