I’m with my boyfriend and this is the first open relationship for the both of us. He doesn’t want to see me after I go to a sex party or go out with someone else. Should I be concerned that this isn’t working for him? I don’t have a similar rule; I’d prefer to see him the next day after he does out with someone so I can hear about or just cause I miss him when we’re apart. He says it’s cause of STIs even though I always ask my partners about testing and use a condom with men and we both already have HSV-1 and HPV. Not sure how to react to his absence after I go out and it makes me feel bad.
It's pretty common for humans to be 'squicked out' by things that aren't entirely rational, but are strong enough that we can't reason ourselves out of it. The disgust response is one of our deepest survival instincts. I got a similar letter about a similar problem last year. So it's very possible that this is just a quirk of your partner's feelings about the world, and not evidence that he's judging you for the way you conduct yourself in this open relationship.
Sometimes, two partners can be in an open relationship and just have different levels on enthusiasm for things like sex parties and active partner-seeking. It could also be the case that he's uncomfortable with something and is channeling that discomfort into a boundary around seeing each other after sex. Your best bet is to ask him!
Make room for him to be honest: ask him whether it bothers him that you're having sex with other people, whether he feels threatened or unsafe by anything, and what he needs from you to feel okay. Let him know that if he really is okay with the way you two are conducting the relationship, he just prefers to leave a time barrier between things, you want to hear that in the affirmative - and if he isn't okay with the way things are, you want to hear about that!
If it comes out that he really is bothered by something else, figure out what to do there. If he genuinely just wants a time buffer around sex parties, figure out how to manage that. You two may have different comfort levels around sharing details and seeing each other after sex with other partners. Your job is to figure out whether there is a compromise you're willing to make - plan to go without his company for a day or so if you want to attend a sex party - or whether this is a dealbreaker for you that needs to be addressed another way.