Confused

I can’t stop thinking about sleeping with other people 🤦🏽‍♀️ my fiancé is amazing we have a 5 month old baby girl. We have a good solid relationship. Lately we’ve been exploring more sexually and have been talking about involving other people in our sex life. Threesomes are a big one we both want to do. I am not ready because I feel like I’d do it for selfish reasons and it’s not fair to him. I love him, and I feel so guilty for feeling this way, but I do. Am I crazy for these thoughts? I’m still pretty young I just don’t know how to go about this or explain to him about my feelings without hurting him.