He is looking now. Why? Not enough sex at home? Take a look at how often you have sex. Perhaps he wants it far more than he is getting.

I would say it crosses guys minds when things are stagnant at home. The difference is some people act on it but most don't.

If he hasn't physically cheated on you (and even if he has) people do mess up, and when its pointed out, if they are worth anything, they'll do the right thing and change. It sounds like in your case he hasn't cheated yet. If he keeps fishing for an affair, I'd recommend seeing a marriage couselor.