You don’t. And why should you care anyway? No crime to be gay, and really none of your business.
Oh, this is another of the perpetual “He is ignoring me, he must be gay.” questions, isn’t it, that come in every single day and almost never get answered cause we are all totally sick of them.
Contrary to what you think, you are not totally irresitable to men. He is not gay, he simply doesn’t like you.
As for why not, well maybe it’s because you assume any problem is the other persons fault and not your own. Check over your own personality first, rather than blaming someone else.