You should be fine then....
"hey hun, I love you dearly and NEVER want what we have jeopardized but I was wondering what you would think if I spent more time with "X" as he seems like a nice guy and could be a good friend as well as a lover.... what do you think?"
This is how my husband approached me. It really helped that he had talked to "X" first and that he had agreed to met me and was absolutely charming and nonthreatening in every way. He told me that I was a lucky woman to have such a wonderful husband and thanked me for allowing him to spend time with him. I was flattered and felt respected. It meant I could let my anxiety go and let them be. They continue to be respectful and keep me posted as to the progression of their relationship. I'm good with all of it and very happy for them.
It doesn't have to be so serious to start off... take it slow and see if this guy is WORTH the effort first. He might not be! If he is then spending time all three of you is essential in my books. It will evolve over time that way. You are already over the "sex" hump, which is usually the biggy!
Anyone want to be friends on Facebook?
Send me your name via PM