The idea is more that most people are somewhere on a spectrum - most lean straight, but might find a person of the same gender somewhat attractive every now and then. Some are straight as an arrow, and never feel any attraction for anyone of the same gender whatsoever. Some are queer as hell and don't ever feel any attraction for anyone of the opposite gender. And then there's the asexuals, of course. But a bunch of people find themselves somewhere on the spectrum, even if they are for all intents and purposes straight.

In a culture where being gay is fine, a predominantly straight person might see someone of their own gender and think "huh, that's an attractive person" and get on with their life without really thinking too much about it.

If this person is deeply religious and of the belief that homosexuality is a sin, or just brainwashed by toxic masculinity, things get more complicated. Moving on is not all that easy when they're confronted with an identity crisis (literally) from hell, and the fact that a bunch of people find taboos extremely intriguing doesn't help either. So they become obsessed.

Which is where homophobia comes in. Because honestly, why the hell should anyone give a fuck what other consenting adults are doing in the bedroom? I think the answer is that they don't - homophobia isn't a response to the actions of others, it's a desperate response to the intrusive thoughts and insecurities going on inside their own brains. In their minds they are straight, so how do they explain to themselves that they keep having obsessive, intrusive homosexual thoughts? The only way out without abandoning their convictions (be it of religion or of their own conception of masculinity) is to direct it towards hatred.

Saying they're "gay deep inside" is inaccurate, because they're not actually "gay deep inside". It's not like there's a flamboyant gay person waving a rainbow flag deep inside of them. They're just normal people who have been mentally repressed to the point of absolute panic at the face of even the slightest intrusive thought. What is meant is that they have sexual urges - weak or strong - that they are extremely uncomfortable with, and that homophobia results from the cognitive dissonance that follows.

It's just easier to say they're gay deep inside, and a bonus is that it probably pisses them off as well. I can see how it's also offensive to the gay community though, so I generally try to use more nuanced language myself.