How did he reproduce with Will? Was Will pregnant at some point?

No, but the first demodog came from Will's mouth, so is like a facehugger thing.

And wasn’t something in Will’s mouth when Joyce and Hopper found him in the Upside Down?

Yeah.

Obviously it wasn't explicit as for example Hannah Baker, but it still a bit creepy.

I was thinking about that monster, but didn't think of it as rape. Besides, given how Henry Creel stopped being a human upon entering the Upside Down, one can argue thatbhe is no longer one.

Stranger Things fanbase; hates on Jason more than the person who not only caused his fall into villainy but is a child rapist and murderer for the evulz unlike Jason.

Well, maybe people didn't think about the implications of that.

Still ridiculous since Vecna is totally the one responsible for why he went evil.

I was refering to the r*pe one, but yeah, Vecna was fully responsable for Jason fall in evil, he was just a normal guy in the start.