Dudes who try to act as though they're not sexually interested in women are either A) homosexual , B) impotent (though can be treated with pharmaceuticals, or C) sexual predators (male feminist).
The thing that bugs me about manga and anime (the ones usually marketed towards males) is that sexuality is treated as taboo. A male character (usually a side one) that shows any sexual interest in a female is immediately looked down upon, belitted, berated and even physically assualted by the females of the story. Strangely enough this doesn't really happen in hentai or manhwa and especially mangas marketed towards women (seriously go read some of those josei mangas).
I suppose this may be due to how a lot of guys feel often. If you're too overt about your sexual interests and you're not following it up with good looks or charm it can be a tough work around to getting laid or even trying to get a girl interested in you romantically. There's just a lot of hurdles like emotional build up, flirting and conversation before a lot of women start opening up sexually. So it can feel like sex is just a huge stop gap when interacting with women.
Meanwhile in the case of women. They don't see it like that. For them they're interested in sex like guys, sure, but they usually want it on their own terms. Often times they're not in a spot where they have to push for this sorta engagement long term, because guys usually very reciprocate easily. So from their perspective this stop gap is non-existent. They don't really have the same struggles that guys go through to bed with a woman when they wanna bed with a guy usually. Usually this is cuz of biological reasons and also a mix of women usually having a wider selection of partners and therefore being more reserved and selective. It's second nature, so for them they don't acknowledge their initial seeming distaste in open desire for sex.
So the difference in how they're portrayed is due to the difference in experiences. And honestly I feel like the guys' one is more true to life. From my younger experiences when I expressed my curiosity with sex openly with classmates SO many girls just ticked me off as a romantic partner. When I switched that up and went for the slower approach their interests started peeking through. So having to reserve yourself for fearing being looked down upon is legitimate as a concern in my eyes.