A lot of people believe the "Nice Guy" or the "Sensitive Guy" is mostly a backlash from the ultra-macho period starting around the 50s that lasted until the 90s.
Though what I am saying is not really to contradict you Vector so much as saying that the "Positive traits" shift around a lot.
Heck it wasn't too long ago that "Geek-lite" was popular. (I say Geek-lite... because they refer to nerds and geeks as the inspiration... but with none of the negative connotations that come with it. They pretty much want a non-dumb person who wears fashionable smarty wear).
But I think what overall was harmful, because Geek-Lite was more of a trend and Nice-Guy was dictated, is when "The kind of guy that is popular right now is this" becomes "This is what guys should be" and starts to be taught in schools, for example (It was an issue for a short while, but our education system was more then competent enough to adjust within like... a YEAR)
the whole celibacy = purity = closeness to god thing?
Remember? It is still going on today in force. People are still trying to marry the ideas of classical purity with secular morality.
It is why, depending on the group, is sexual liberation either a negative or a positive as well as what forms count or not.
Though for men where that issue affects them is... rather complicated. As they have both a purity and counter-purity ("You better be having sex") culture going on for them at once, but a more strong counter-purity culture. Sex is somehow equated to a problem solver and lack of sex is seen as a problem.
So for men it is often more pressure to try to conform to a culture that asks them to have more sex. (but honestly, I think after you grow up... like after highschool... it stops mattering... but that isn't supported by anything but my own observations. It is why many people believe "The 40 year old Virgin" is actually a parody of this attitude)
Mind you it is an interesting subject... I should explore it more, but the last time I looked into this subject it was more about how men are taught to be controlling and abusive by being forced to play into macho stereotypes and not to allow any vulnerability thus creating a caustic lifestyle... so it is rarely a happy subject and rarely spoken in observational tones. (It was an interesting video for something in the 90s.)
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Mind you right now I think the "Guy that is in" is the: "Self-made man who is distant, semi-abusive, and commanding who could have anyone but choses you, who you can totally change to be a more caring individual"... but that is just in Romance Novels.
I HONESTLY want to know why "Fixer uppers" are so dang popular in Romance...
I wouldn't know what the "Guy that guys want" but... honestly... it is probably around the same thing with less "Change them" in there. Which I guess gives the guy on guy a "Caretaker" sort of role oddly enough... but I don't read gay romance so I have no idea... this is pure biased assumption based on misanthrope.