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Maybe there just become too many strings and other things connected to it and they decide it just isn't worth the work/effort with the high potential downside. IMO many women use it as a weapon/tool/bribe/penalty and maybe the men are just tired of being manipulated that way.
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Exactly. I'm sure it's not an "overnight" thing, but something gradual over time that occurs until one day sexual pleasure with the partner becomes anything but pleasurable. (Again, women can probably relate more to this emotionally.) It's like you said, they weigh the upside (sexual gratification) with all the downsides (e.g., possible rejection yet again, manipulation, no actual pleasure aside from "emptying the tank") and figure its just not worth it. They figure why bother when they can get a lot more pleasure from a Victoria Secret catalog.

Psychologically speaking, men want to make love to a woman - not an ugly soul wrapped in a female body.