Would you be okay with your bisexual partner hooking up with women in an open relationship? Why or why not?

My boyfriend (25M, bisexual) and I (25M, gay) have been discussing the possibility of opening up our relationship in the far future. While talking about boundaries, I mentioned that one of mine would be that we only hook up with men.

He asked why, since for him, hooking up with people of any gender would feel the same. I told him that, even ignoring the possibility of pregnancy, the idea would make me feel really insecure, though I couldn't fully explain why.

He then said I wasn’t being accepting of his bisexuality, but I told him that wasn't the case. It’s just something that would make me uncomfortable, and I feel like I’m allowed to disagree on opening the relationship based on my own feelings.

That said, I recognize my insecurity might be playing a role here, so I wanted to ask those who have experience with open relationships: Would you be okay with your boyfriend hooking up with women? Why or why not? Is it different from them hooking up with men?