Most people, particularly on the left, define “being gay” as “wanting to have gay sex.” The conservatives who push the idea that “being gay is a choice” tend to operate on a different paradigm - to them, actually having gay sex is what makes you gay. Someone who wants to have gay sex, but only has sex in the confines of heterosexual marriage is straight, even if they have to pop Viagra like M&Ms to get it up with a naked woman. Someone who has adopted this framework would, generally, acknowledge that the person didn’t choose to be attracted to men, but they did make the choice to act on that attraction, and that’s the “choice” they’re talking about when they say “homosexuality is a choice.”
For fairly obvious reasons, this way of conceptualizing sexuality is extremely attractive to people who were raised in conservative cultures, and are struggling with same-sex attraction, because it means they can be “really” straight despite what their libido is telling them. So people who are closeted end up significantly over-represented in this demographic, leading to a lot of the Freudian statements like, “If we didn’t oppress people for being gay, everyone would choose to be gay and humanity would die out!” Which feeds into the impression that most people spouting homophobic ideas are homosexual themselves, but I don’t think its actually anywhere near the majority.