Thank you for the thoughtful response!

So is it...the sexual conversations? If so, is it the openness of their sexual conversations or the fact that their views on sexuality are so different from your own but so similar to each other's?

The odd thing is that he doesn't talk this way about himself. He is very straight laced and unskilled at articulating aspects of his own sexuality. It find it puzzling that he knows so much about hers. One of my ex's and I would go watch their pal do burlesque shows so it's not like I'm entirely prudish, you know? Maybe if he were more open about sex then I wouldn't be so weirded out.

Is it the closeness of their relationship? If so, is it that you feel she is overly dependent on him or is it that you feel like he is prioritizing her over you?

Yes, and the former.

I think I understand of the roles of friends differently. I see them as companions, not supporters. My friends and I just joke around, go places, and watch movies and TV together. I like it.

The only people I am comfortable spilling my guts to is my SO and therapist. Being able to care for my partner emotionally is such a special privilege to me. Certainly this contributes to my resentment and confusion. Like "wtf lady, you already have a bf."

You need to get to the bottom of what the issues actually are for you, then you can have a conversation with him about why they make you uncomfortable and what you can do (as a couple) to resolve them.

The problem appears to be that I am a (functionally) codependent dinosaur. I'm no sure how to discuss this with him without making him feel controlled and infringed upon.