Just because she's enthusiastic about his penis doesn't mean that he knows how to please her in other ways.
To me sex is about SO much more than PIV sex! It's about two people who love each other, care about each other, spending some time bonding in ways that are specifically unique to them! It means the chances of my doing things which I do NOT enjoy are in the negatives since this other person can fulfill that need for him. It means personal time for me to recharge my batteries or get some things done while he's away that he doesn't enjoy or which take time away from him when he's home.
When my boyfriend does have sex with another it is their special bonding time (not that they don't bond in other ways as well). I do still have issues (and they ARE my issues, not his or anyone else's) with his being with someone else in a sexual way. I just have to remember that it is ME he chooses to come home to each and every time it happens!