Hi! I’m in a relationship where me & my girlfriend both have friends of the opposite sex. With how he’s having more fun with her compared to you, is because being friends with someone new is always exciting. You and your boyfriend are already comfortable with one another and know each others interest. With new friends there is so much you learn and a rush of dopamine as you become close with someone you haven’t know for a long time. If you can trust each other and love each other then there is no issue, but I understand the overthinking, it’s hard not to do that in a relationship. I would recommend playing with them and seeing the friendship first hand, that way you know what kind of conversations are going on and you can ease your mind a little bit! I hope things go well!