Note: I did NOT come up with this theory. I read it somewhere years ago and I forget the source. If anyone can remind me of the source, I'll link to it in the article.
I was out with a friend and she remarked "I don't know why guys get so bent out of shape to see two guys kissing. They like it when women kiss, but get so threatened when guys kiss. What's their deal?"
The following was my response:
It comes down to these men hanging their entire self-identity on the traditional view of romantic relationships where men are the predator and women are the prey. Men hunt and consume, while women are hunted and consumed. Men are the strong predators and women are the weak prey.
The reason it is not threatening to watch women kiss is that prey can consume each other and they are not a threat to the predator. Each woman is playing the part of the prey and is being consumed, so men can still feel strong and unthreatened as the predator.
But when men kiss each other, that means that one of them is the prey and the other is the predator. One man is being consumed while the other consumes. To a man who clings to a traditional view of male-female relationships, it is disturbing to watch a man play the weaker role as prey, as being the one who is being consumed. It makes the man worried that he may not always be the predator because of his maleness. Watching two men kiss tells him that he could be the one being consumed rather than being the consumer, he could become prey, and this feels like a violation of the natural order of things.
Similarly, the man who clings to his predator title will feel threatened by a woman who hunts him. A woman who is self-assured, strong, confident, and who take the power of the predator will threaten a man who feels entitled to be the predator. He is no longer the predator by default because of his gender: he will need to put effort into his role as predator and he may fail at this task, making him the prey, making him weak, making him be the consumed.
In conclusion, men who are uncomfortable with expressions of non-heterosexuality do so from a fear of losing their male-entitled social position as predator. These men are afraid that they'll be treated the way that they treat women.