Married but wanting a girlfriend

I’ve been with only one person, my husband and I got with him when I was 16. He was my first boyfriend and I married him at 18 and had a baby at 20. I don’t regret it at all and I’m so glad I married my best friend and have a wonderful baby with him. But lately I’ve been wanting a girlfriend. I’ve always been attracted to women, even as a kid all id have is girl crushes and maybe a boy crush once in awhile. I’m always looking at girls and fantasize being with them. I’ve talked to my husband about this and he said he’d be ok with my having a gf but I think that’s only bc he thinks we’d have threesomes with her. I don’t want that. I just want to be with her. But I don’t want my husband with anyone else? It’s unfair to him and idk what to do.