The easy answer is an old rhyme:
"Higamous hogamus
Women are monogamous
Hogamus Higamus
Men are polygamous :-)
the harder answer is: you can only do what you can do. That is you can be all that a man could want, loyal, loving, nurturing and that kind of guy will take what you give and still look for outside love. You cannot change him, really. You *can* change your tolerance for infidelity and respect yourself by setting boundaries. And mean them. if, after you have told him, he disrespects you then admit he did and make the hard choice. All that said, if this is not a path you feel you can walk at this time, speak to Lady Mimi and see what her resources can do. In fact contact her first. She's had a lot of experience in things like this and can be a great help. Everybody deserves some love in their life.