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some gay men would swoon with disgust if they had to touch a woman's private parts, while others are quite capable of having sex with women.
There is a common tale of married men losing their ability to perform with women as they get to middle age, particularly if they become emotionally involved with a boyfriend.
There is a whole world of emotional intimacy lost with a bisexual. I think the differences between gay and bisexual are part of the myriad of genes that control mating behaviour, and affected by the hormone changes in the process of maturing. and the main switch, that magnetic pull - attract or repulse, the sexual orientation remains steady. Like which direction your feet face doesn't change.
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I agree with you that there is a spectrum between attraction, neutrality, and aversion, but I think that can be true for everyone, regardless of sexual orientation.
What you are describing, (gay men who are able to tolerate sex with women), in my opinion is not the same thing as being bisexual. A bisexual person is genuinely, enthusiastically, passionately attracted to members of both sexes, both physically and emotionally.
But since we seem to conceptualize the idea of bisexuality differently, we can just agree to disagree on this point.