It is important to make sure responsibility for oppression is placed cleanly on the shoulders of those doing the oppressing. Saying that homophobes are probably gay makes this harder.
It’s a common trope, and to be fair there have been a lot of instances of homophobes actually being gay. That doesn’t mean that it’s true or good to say. Gay men being beaten for being gay typically do not feel much same-sex attraction emanating from the homophobes who assault them. A list of gay homophobes is no more proof that most or all homophobes are gay than a list of men who like pecan pie is proof that most or all men like pecan pie.
(Pictured below: Aaron Schock, an anti-gay former U.S. Representative who was outed as gay.)
Beyond being factually shaky, it is also just not good. It’s not good because it shifts the responsibility for homophobia off of straight people and straight culture. Most cultures have a consistent habit of forgetting gay people and elevating straight people. It’s undeniable that the working assumption is that people are born boy or girl and that boys like girls and girls like boys. It’s also undeniable that there is rampant homophobia. Homophobic gays are victims of a homophobic culture prior to them being perpetrators of it.
Sometimes, “homophobes are gay” takes the form of an insult. Instead of (or alongside) being a way of pointing out hypocrisy, it veers into “you take it up the butt [and that’s bad]!” territory. At that point, it’s not even vaguely progressive; it’s just straight up homophobic. There’s nothing wrong with taking it up the butt, and I will die on this hill.
That being said, what’s up with the people who are gay and homophobic? It’s not enough to say that they are self-hating, although that is typically true. Homophobia is learned, not innate. Their homophobia is an instantiation of a cultural meme. The proper response to a gay homophobe isn’t to mock him for being gay (seriously, don’t make fun of people for being gay) but to offer a better path forward. “The gays will be here to welcome you if you change your mind” is far more tempting a response than “lol i bet u like dick.”
Of course, growing up in homophobic surroundings does not absolve people of the harm they cause by perpetuating it. Aaron Schock voted repeatedly against pro-gay laws and for anti-gay ones. Even though he came out as gay, he has to actually present something resembling an apology before a warm welcome can be justified.