i put Attention. I think thats why any one cheats. Your eeds aren't being met...and i don't just mean sex. Sometimes its taking an interest in the need of the spouce. sometimes its more than that.
My guess is all of the above, depending on the guy. I answered "nature" because I think some men are just wired that way-it's probably just in their nature to have a wandering eye. On the other hand, thankfully, some men are NOT wired that way.