This sounds like a Doujinshi plotline

‘cos raping a male as a female is more socially acceptable?... hahaha?...

Yeah, not gonna touch the rape factor. An erection while a male is asleep isn't any indication of what type of dream they're having. It's a normal thing that occurs 3-5 times a night in healthy males. https://www.menshealth.com/health/a19540431/nighttime-erections/