What do i do?

Every so often, my boyfriend asks me for a threesome. He says the last time I showed him I really wanted him was when I kind of went along with it 2 years ago, but the person we were talking to stopped replying. I never wanted to do it because i dont feel comfortable with doing that. My spirit wont let me. He’s done it before with an ex of his, and with another ex, he stated she wasnt comfortable with it and that was that. I keep telling him no but every so often he circles back to it. The last time I told him that with me growing up and putting God into my life, I just cant.

What do I do if he just never stops asking? We’ve been together for 4 years.