In other words, I don't think there is a very good scientific explanation why a man should remain monogamous, or why willing people, should not engage in sex.
Science is the wrong place to look to address these questions. Politicians don't consult scientists when they legislate on these kinds of issues.
I would say that these are matters that develop in the culture. Scientists have relatively little influence on cultural trends. The entertainment industry is far more influential than is science.
So, for example, when we prosecute a 30 year old teacher for having sex with teen-age boys, are we imposing our religion on them?
No, we are enforcing our laws when we prosecute.
As for the basis of those laws - that goes back to moral and ethical questions.
If a faculty member at my university were to date a student, that might cause a few eyebrows to be raised. But it probably wouldn't cause serious concerns. If, however, the faculty member were to date a student enrolled in a class he is teaching, that would be looked at a serious problem. The difference here is that a faculty member is in a position to coerce a student enrolled in his class. The 30 year old teach having sex with a schoolboy would likewise be considered coercive. Statutory rape laws are there because of questions of coercion.
How about if a 30 year old man sleeps with a 15 year old girl?
This is statutory rape in most states, and the concerns are with coercion.
And here is the kicker. If so, does science inherently lead to amorality in terms of sexual behaviour that is non-violent?
No. Such moral positions don't come from science.
If you want to find someone to blame for current sexual behavior, I would suggest that you look at the political right. For they emphasize market forces as the correct forms of control. If you can get rich selling porn, that's okay with the right wing free marketeers.
Don't look to science as a cause.