Weird thaught

So I had a wired thought my boyfriend and I lost our virginities to each other and we plan on staying together. Everything is going well and all, but if he would to ask me to sleep with someone else for a sec like only once because we have never been with anyone else I don’t think I would say no, but only once. It wouldn’t be cleating because he asked. To clarify my boyfriend would never ask and if he ever would want to do that he would just have us do a three sum. Have any of you lady’s done that.