I think the heart of your question is not why men are so sexually immoral (keep in mind - for every MAN fornicating or committing adultery there is a WOMAN doing the same!); but rather, why do men seem to be so much more focused on sex than women?
Biology. It’s just simply the way God designed us.
Men, being designed to be very visually stimulated, are naturally (especially in this day) going to be more easily “turned on” than women.
Women are designed to be more turned on through attention and emotions. The physical turn-on (arousal) follows.
It’s important to understand basic biology here and not (as the feminists would like you to) lump men and women into the same category when it comes to sexual needs and desires.
Ultimately, BOTH men and women are seeking true intimacy when using the vehicle of sex and their bodies. They are seeking that unconditional love and affection and acceptance that can ONLY truly be achieved within the marriage bond.
When it is outside of marriage - that intimacy, while “looking” the same (sexual act) is a counterfiet. It is never a true and authentic love of completely, without reservation - giving of oneself.
So men are no more “immoral” when it comes to sex than women.
Men ARE however - wired differently and more easily “aroused.”
Thus - they, more than women must take greater care to practice self-control.
And this is why traditionally, it was the GIRL who was expected to “say no.” Because… it is EASIER for her to do so. Just biologically speaking.
Men are like microwaves - push a button and they are quickly ready to go!
Women are like slow cookers - it takes time and patience etc. and then they too are ready to go!
It’s just the way God designed us.