I think it's very simple. If you spend all day thinking about gay dick, your body is going to react. Normally only gay mascs and horny women spend all day thinking about gay dick. Straight men don't have a reaction to gay dick because they don't condition that response. They don't think about gay dick for long enough for Pavlov to kick in. Homophobes condition themselves by constantly thinking about sex (hot; unconditioned stimulus) and gay (conditioned stimulus) at the same time. If they just quit thinking about gay sex all day, their fetish would go away. But that would require being less homophobic, and they can't. Then they get worried that they're gay, because they like dick, so they assume being gay is what they have. And since what they have sucks, they assume being gay sucks, which makes them more homophobic.

A similar thing happens in POCD, or Pedophile Obsessive Compulsive Disorder. The fear of being a pedophile conditions an association between children and arousal, which self-reinforces in a vicious cycle. Fortunately, most people with POCD are not fascists, so they're capable of having a normal response to it in which they don't proceed to deliberately indulge in the association. In order for someone to be homophobic and suffer the same thing with gay thoughts, their judgement has to already be compromised, which produces a more toxic reaction.