I think with this it can also help to think about how your own physical response has probably fluctuated during sex. If you aren't someone with a penis, then you probably are someone with a clitoris: during sexual activity, including masturbation, you may have already experienced that your vulva can be puffier (that's in large part about the internal structures) than not, or your clitoris can be harder or larger sometimes (it has erectile tissue, too!) and softer or smaller others. If you have a vagina, you may have noticed that you don't always self-lubricate, or have times during sexual activity where that's happening, and times when it isn't.
Point is that for all kinds of bodies, during sex how our bodies are responding is usually variable, and often that doesn't mean anything except that we have a body, because human bodies are just unreliable with certain functions like that. Penises being erect or not isn't even something that often falls under what some people call "erectile dysfunction," because there's not anything dysfunctional about our physical responses varying during sex like having an erection then not having one. That's common and normal.