My husband says he won't have a vasectomy in case he wants to have kids with someone else one day. Help?

My husband won't get a vasectomy in case we split up one day and it makes him less attractive to future mates. I don't know how to deal with this. Before we got married he agreed that when we were done having children, he would take care of it. Now he says no way. Can anyone suggest what I can do or say to convince him to go back to our original agreement? Our marriage is great, or so I thought. He says he isn't planning to leave. I feel this is suddenly a question of commitment although he disagrees.