This is a view very common in the U.S., and with all due respect, I think we as a society need to grow up. So someone promises never to have sex with someone else for the rest of their life and doesn't keep that promise. So what? It's unreasonable to expect people to keep a promise like that to begin with.

If we're going to view marital fidelity as an essential quality in a person, then we'd have to conclude that Martin Luther King, FDR, and JFK were unacceptable. And there's no reason why someone can't be a poor spouse or parent and be an excellent president, prime minister etc. I think the prevailing attitude on this issue (in this country at least) is stupid and childish.