I think if your wife says she's happy with your cock above all others then take her on her word. She has the experience with other cocks so she should know what works for her. Don't let your insecurity ruin what you have.
Having said that it seems your wife likes to fuck others from time to time, so as long as that works for you both then go with it. Does she know how you feel in a MFM situation & is she fine with you being a sub? Maybe you should let her know this is what you like in case she has concerns of how a more assertive male in the room will impact you.