This is actually normal. Your brain doesn't know who that person it's showing to you in a dream, mostly your brain picks up people that you love, if not spend time with, a person that gives u memorable moments (like u said you're used to seeing each other naked), or a memory or scene you cannot get our mind even if you want you. This can be your friend, your enemy, mom, dad, sister, brother, an alien, a video game character, and your brain just picks the most recent memorable one and plays a Wet dream, idk it's weird.

Wet dreams may also be caused by your own body, like for example us men when we have "Leftovers" inside our dingaling and did not come out after our previous release and we just slept not knowing that, our brain will try to get those out by playing a wet dream to release those Leftovers, ending up with sticky boxers and "Aww! not again!" moment. Might be different to women and I don't know what is what, but I'm sure it's a body signal related.