Threesomes with boyfriend

I’ve been with my boyfriend for 2 years now, we are sexually active but he is wanting to do a threesome with another girl. I don’t mind the idea of threesome but because we are in a relationship it makes me feel like I’m not enough for him and I also know that I wouldn’t be comfortable watching my boyfriend penetrate another woman. I’ve tried telling him that I feel this way but he thinks I’m just being immature. I’ve also asked him if he’d be able to handle it if it was the other way round and a male joined us and he said no. So overall I think a threesome during a relationship isn’t for either of us but he doesn’t seem to get that. Am I overreacting though?