Im alright with my partner having other romantic relationships, but I’m not fond of the idea of him having sex with other people. Is that wrong? I’m worried I’m being manipulative.. but he seems ok with it.

If he says he’s okay with it and you don’t believe him, dig into why. Is it because he has a habit of saying he’s okay with things, but really not being okay with them, which can lead to passive-aggression and resentment? If so, you need to talk this out with him and try to find a safe way for him to be open and honest with you, or reconsider whether you can be poly with someone who doesn’t tell you the truth about how he feels.

Or is it because you are having a hard time believing him despite no evidence that he’s not actually okay with it? If so, you need to do some introspection and figure out why you don’t trust him when he says he’s okay with it. Did you have a previous partner who wasn’t honest about their feelings? Are you projecting your own discomfort and assuming he shares it? 

Or, if your goal is to get past this and be okay with him having sex with other people, that will take some self-work as well. Why are you not comfortable with it? What is your worst-case scenario? Best case scenario? What are your fears and concerns? What can he do to help alleviate those? 

But ultimately, if both of you are okay with a situation, and you’re both being honest with yourselves and each other, it’s not wrong or manipulative. Don’t feel like you need to change something about your relationship if it’s working for all parties involved.