NTA. By itself (which it's not, but wait for me to reach that point), stepbrother/stepsister porn is not a red flag.

There are a lot of people that don't like it, but it's currently super popular because it's such an easy fetish to cater to. I've seen multiple Reddit comments about people who just mute the videos, because aside from them calling each other stepbrother and stepsister, it's pretty much exactly the same as regular porn.

Further, I do actually like it because it's something slightly "forbidden," and it feels like the characters already know each other, which is something I prefer in my porn. However, I have zero desire to actually sleep with my stepbrother, because the things I find attractive in porn are not (necessarily) things I find attractive in real life.

That said, him going through your drawers and walking in without knocking? And badmouthing all your boyfriends? Putting that all together with the porn is pretty creepy.