Truthfully, I have never seen a black person in an actual relationship with a girl for a period longer than mere weeks, though I have little experience to go off of.
My high school was in a town that is 99% white, and my own home town is some 99.5% white. There was, during all 4 years of high school and EVERY SINGLE year of elementary and middle school, with 1500 kids in my high school, not a single Muslim kid, nor a single (practicing) Indian or fundamentalist Jew. None. Well over half were Catholic-raised and almost all of the rest non-denominational Christian.
In any event, it'd be idiotic to think that people of other races cheat less by virtue of being a different race. In fact, I would imagine that infidelity is rather uncorrelated with the main factors that correlate with race (the top being social status). Poor or rich, doesn't matter, they're all filled with cheating pricks and whores.