If he cheats once, leave him. There is nothing to it. You don't need to fight for somebody who will probably infect you with HIV.
Let's be realistic - he will not chose to date one, but many - how much of your health are you ready to sacrifice?
Do you need this sort of love?
And love is not war, it is not something you can win.
It is, or it is not.
That's how I see it.