I say no, no raping or coercive sexual activity under any circumstances. Besides, if you’ve really got a sustainable breeding population of many hundreds or thousands of individuals, you’ve probably got enough infrastructure left that there’s a source of viable frozen sperm around somewhere.
What it comes down to, IMO, is that any situation where there is literally only one individual on whom the survival of the whole human species depends is a situation that’s too precarious for the species to last more than a couple reproductive cycles anyway.
If the apocalypse has left thousands of fertile females and a sufficiently robust social system to sustain them in living and reproducing, then I guarantee you it’s also left at least a few males willing to reproduce.