My headcanon is that men are receptive to lesbians because they have found a way to make it be for their pleasure. It's the idea of having 2 women at their disposal. So the women still exist for their sexual pleasure.

On the other hand, men don't exist for the pleasure of other men in our society so they struggle to conceptualise why a man would want to be with his fellow man. And because it is generally unsanctioned to kito someone, they feel like they're defending masculinity by attacking the "wrong ones"