There are plenty of good-looking men in committed relationships. You're reading the relationship in reverse. It's not that "men who hop relationships look all that better". It's that better-looking men can hop relationships.
That being said, the desirable men are either committed (unavailable) or available because they can hop and choose to do so. So only a minority of desirable men are available. This is in strict contrast with the large majority of young women, who can almost always hop relationships should they so choose. Since overall there needs to be gender equilibrium, females are sliced by age to a much larger degree, allowing the minority of desirable men of all ages to pursue the minority of younger women.
The asimetry is caused by the fact that females are selective and males are partner-maximizing. It's not good or bad, it's a sociological truth.
Also real sociologists do not claim what you claim. They have a lot more nuance into it and in terms of marriages, talk a lot a lot more about economy and it's affects.
Your version sounds like something that emerges on chat when people cherry pick half understood studies, not what sociologists says.