Boyfriend wants a threesome but... (Please help me)

Before you read, just know I am an insecure, self loathing piece of shit

I’m 19 and my boyfriend is 20. We’ve been together for 4 months now and I think we connect well. I’m madly in love with him and I’d do pretty much anything....

Except having sex with other people. I just can’t. I see sex as an intimate act so I don’t feel comfortable with other people involved. I’ve always felt so alone too and I just want someone to cherish me.

He said he loves me, wants to marry me, and be with me forever but he would like to have threesomes and wants me to have sex with other women (I am bisexual, and yeah this is exactly what he said)

I expressed that I don’t want to do it at all and he said “that’s okay, I love you so much and I don’t want to hurt you ever. I am fine with being with just you”

My rational self says okay girl it’s fine everything is okay but my irrational self is screaming “he loves you sooo much but wants to have sex with other women? What the fuck? Why am I never enough? Why can’t he treasure me like I treasure him? Why?”

Idk what to do. He seems very genuine and wouldn’t do anything behind my back but I am still upset. It is what it is, I know that, but I’m just so sad.

I’m always fighting myself. Everyday is a nightmare