Would you be comfortable with your SO changing with the opposite sex?

I am trying to be rational about this but it makes me uncomfortable. My boyfriend is on a sports team that just got a woman player. She changes in the room with them. I am trying to be understanding that they aren’t stripping down naked but I still feel uncomfortable with it. I haven’t told him that he can’t go or play because I want him to play the sport he loves but I can’t help but feel uncomfortable. He told me that if I really wanted he wouldn’t play but that’s selfish. How can I ease how I am feeling? Would you be comfortable with it? I am trying not to overreact just trying to see where others stand.

Edit : I would like to add that this is an adult league. He says that it’s not what I think and that no one gets fully undressed. But I don’t know that for sure. To me that is kind of besides the point anyway, changing with the opposite sex, despite what they leave on (whether it’s yoga pants or just a sports bra) is inappropriate. Especially for those in a committed relationship. He says he would rather not go if I’m going to make a big deal about it because he doesn’t see a problem with it.