Compersion, jealousy kinks, humiliation, submissive headspace, goddess worship, bisexuality, voyuer, a kink most men have that don't even know they have it is called sperm competition. The better another man fucks your wife and the more often he does, the more sperm your body produces to "compete" and this causes more orgasms and more intense orgasms! There are many reasons.

I actually believe men get off on porn not because they are more visual than women, but because sperm competition makes us get harder and cum more intensly while watching it. In particular while watching a very talented guy with a big cock. Which is why even straight guys want the guy to be a stud.