My husband wants a 3some.

Okay so before I met my wonderful husband, I had 2 threesomes. (Both with two men) and I've grown out of it. Yeah it's hot but I'd rather have sex with my husband not with another person I don't know. I've had sex with girls (I'm bisexual) and my husband knows. He likes it when I check out girls. He keeps bringing up the idea of having a threesome with another girl. He says he just wants to watch us. But I feel like I would get so jealous and upset. I've talked to him about this and he understands it would make me sad and insecure. But he keeps bringing it up. I know he wants to respect my comfort and boundaries but I know he really wants to. I don't mind having sex with another girl but I feel like if I see him even look at her sexually I'm going to get pissed off and hit her or leave the room crying. Please help. Idk what to do.