Men are not programmed to be in monogamous relationship, to sexually be with and commit to 1 woman forever. It is social construct designed by the powers that be to help society produce and produce and produce. If a man has a family, he is going to go work his ass for them and produce for society.
Yet the male brain craves sexual variety. That's why porn is such a big big thing among men. It's the sexual variety that keep men coming back to it. Suppose there was only 5 porn scenes in the whole of the internet. Eventually men would get bored of it and go find real girls.
Back to the OP, yes men like watching sexy, young, hot girls when they are in a relationship. Men don't really get into a relationship for the sex, a man can get sex anywhere now. Very easy. They mostly get into the relationship for the companionship, for the not being alone, for the being invited out on weekends and do couple things with other couples. Since they don't want to cheat on their gf, and need sexual variety, porn is the quickest and safest and most fun way to get the release.
Now if you go to the marriage aspect of it, back when, all married men had at least one hot sexy mistress under 30 they could smash on the side. Yet the powers that be realized "uh oh, this is not good. This gives men power and takes away women's power, which is not good for us. Lets make having a young mistress the worst, most despicable thing in the world. That way men can stay married and work their ass for society." So then married men go to adult entertainment as well.
Just on a side, when I go to business lunches with older women, the one thing they tell me is their man "can do anything, but can't cheat." That's the only thing the care about, the cheating. Why? Because that's where their leverage is the lowest and the man's leverage is the highest.