Heh, I am actually surprised that on a forum which generally interests open minded people there is still like 30% of people agreeing with my assumption. Can you imagine what the general population thinks lol.
Since I asked the question it is fair that I put in my 2c as well, am a guy though. A lot of girls seem to like gay porn, but a lot of them also find bi men sketchy. Probably because they attribute it to promiscuity, diseases, or reduced 'alphaness'. One of my ex'es was super into exploring her lesbian side, got a chance with MFF, and already with clothes off while fooling around the guy tells the girls he was bi. My ex right away walked out lol. Other friends would stop dating guys once they found out the guys were bi.
Personally I think the assumption holds true for a decent percentage of girls and definitely a HUGE percentage of men. Yet IMO bisexuality is a spectrum, very few are 100% straight outliers on the bell curve. I am bi, but i find attractive very very few men, and all of the ones that I do like have feminine features. i.e. facial or body hair? barf.. Heck, even kissing a guy that I do find attractive makes me feel uneasy, but doing much dirtier things to them... hello boner. On the other hand, I would happily bang any female. It is bizarre, ugly girls can be a turn on, but ugly guys though, ewww. My GF on the other hand who knows I am bi, has been with hot girls with me in MFF but it is just not her thing, she does the bare minimum to them out of courtesy but no more. With MMF though, she is super into it. And not just because she is getting double D, she genuinely likes watching the two dudes with each other while touching herself. You would think with all this reinforcement I would not give a shit, but there is still some buried fear in me of being with guys in front of her.
It is also fair to mention how does one define manliness? If it is being stronger then other men then gay guys are probably more manly on average in the 30+ crowd. At least in my experience which is limited to edm underground clubs: a lot of gay guys are beefcakes in the older group while hetero guys just let themselves go. However I would define manliness not how strong a guy is but how appealing he is to women. Which is stupid, but logical and reinforces the original assumption. If one says manliness is not giving a shit about how appealing women find you, then lets extend the argument that all women find you unappealing. How do you reproduce? Going super alpha, club them and drag them to your cave?