It is victim blaming, we are not the reason homophobia exists or that people become extremely homophobic. It is not oppressed people that cause their own oppression, we don’t in some way “deserve” it.
Some aspects of the identity of sexuality may be related to physical and things we cannot control, but at the end of the day it is an identity. If they do not see themselves as gay, they are not. It is not for you to assign an identity to someone, even someone you don’t like. Even if someone might identify as gay outside of these power structures, in here they aren’t. Even if they would be gay, they participate in our oppression because being straight is beneficial to them, not because they “are secretly gay”. If they did homophobia because they were “ashamed” it wouldn’t be helping so many of them get into positions of power
Members of the oppressed group becoming zealous defenders of their own oppression in exchange for a slightly improved position in the hierarchy is a very real phenomenon, and I don't doubt it happens with sexuality/gender-related oppressions too. But, obviously, straight people produce their own chauvinist homophobes too, so being a homophobe is not a great indicator of being gay. So the question becomes, what purpose does the belief that most ardent homophobes are secretly gay serve for the oppressor?
I think the main reason is that extremist homophobes inevitably break social norms, which creates the need for an explanation for their existence. The straight moderates need an answer that is individualistic, because a societal answer would likely involve their tolerance of homophobia and opposition to concrete acts of gay liberation as a main cause. Homophobes being insecure gays is a great explanation for them because in addition to shifting the blame away from them, the praxis that it implies is what they are already doing to maintain the oppression.