After further discussion with 2 Japanese experts (the N1-guy and someone who's studying it and married to a Japanese woman), we all three agree that the best translation for 寝込みを襲う is probably that 'he forced himself on her'.
Basically, 'rape' is too strong of a choice, because the way it's portrayed doesn't indicate some violent, desperate struggle. But it's also clear that he overstepped some boundaries and is a pushy idiot. So 'forced himself on her' leaves enough ambiguity without letting him off the hook too easily, until the anime reveals more details about it.
That'd be my last posting about it ... until the anime continues that lol