A theory I've heard many times but it really is one that lacks any evidence. There is no real reason to assume Paul was gay just as there is really no evidence to say he was not.

I believe most homosexual advocates want to read into friendships between men with some romantic element that non-homosexuals wouldn't assume exists. For example there is the so called "embrace" between David and Jonathan used as evidence of a homosexual relationship which is laughably silly in light of the fact that David basically had a harem and actually got a guy killed so he could steal his wife after he saw her bathing because he wanted her.