bf won’t stop talking abt adding a girl even though i’ve said no

i know this isn’t the place for this, but idk where else to get advice. My(23) bf(22) keeps trying to add a girl to the relationship. I’m bisexual and he claims that it’s the same thing as being poly, but it’s not. how can I explain that’s not what I want. I’ve told him I can’t love two people at once and he called me weird. what else can I say? i really love him and we live together, have pets together, know each others family and talk about marriage and having children too.