[Obvious trigger warnings for rape culture and homophobia.]
Many of the common arguments against homosexuality tend to strike those of us on the Left as so completely unrealistic that we assume they must be barefaced lies. In particular, there seems to be a fear that homosexual necessarily means homosexual rapist. This is usually explained under a theory (on the Left) of a theory (on the Right) of "moral degeneracy", assuming that a person with one "bad" trait must have all traits that are considered bad.
I wish to argue for a different theory, which is that many homophobes, especially male homophobes, are starting from the assumption that all men are rapists.
Much has been written about rape culture, a social phenomenon of normalizing and excusing rape. A person thoroughly steeped in such a culture, who has internalized its norms, would naturally have a greatly weakened distinction in their mind between sexual desire and sexual acts.
We see evidence of this conflation in various popular tropes: the man who wakes up after a night of drunken partying to find himself in fuzzy pink handcuffs locked to the bed of an obese woman, looking horrified (assuming that she won't unlock him at his request); the man fleeing a lustful undesired woman, who chases after him; in general, the use of unreciprocated desire to imply unwanted advances, and of unwanted advances to imply nonconsensual interactions.
When viewed in this light, the Gay Rapist trope makes a lot more sense. It's not that homosexuality is assumed to carry with it various other perversions as well. It's that, according to the model of normal human behavior as propagated by rape culture, if you start with a Standard Default Human, add homosexuality, and make no other alterations, the most natural result is a rapist.
According to this theory, the only reason for homophobes to object to homosexual rapists is that they suddenly find themselves in the unfamiliar position of being potential victims of rape culture, rather than its beneficiaries.
Is it an accident that rape apologists and homophobes tend to congregate in the same social and political circles? Maybe. It's only one bit of information, after all -- Left vs. Right -- but there does seem to be a natural concordance between the views. If we assume that most people model homosexuality as basically the same as heterosexuality but with different genders, then we'd tend to expect pretty much the beliefs that we in fact see. People who think rape is normal expect homosexuals to be rapists, and people who think normal people don't rape expect homosexuals not to rape.
This model certainly doesn't explain all homophobia, but I think it could plausibly be one of the major forms or components.