It's one thing to say that portraying certain behaviour on TV or in games or whatever can normalise that behaviour, but Billy seems convinced that simply seeing homosexuality depicted, in any way or any context, can actually alter your sexual orientation. Nobody was ever "turned" gay by anything, just as conversion "therapy" never turned anyone straight. You could use coercion, guilt or fear to stop people acting on their orientation, but nobody ever had their orientation changed by anything external.
You can criticise the way gay people have been depicted in popular culture for the last 30 to 40 years. Even so-called "positive" portrayals of homosexuality have been very harmful to gay people themselves - a whole generation of gay men in particular grew up thinking that in order to be properly gay you had to be a shreiking queen or a San Francisco leather bondage faggot, and have loads of unprotected sex with strangers in public bathrooms. This not only caused direct harm to gay men who got diseases or got sexually abused, it also harmed the acceptance of homosexuality in wider society because it made gay people seem very different from straight people in ways that they weren't. It also prevented people from coming out, because they didn't feel the need to dress like Rob Halford so they wondered if they were really gay or not, or didn't want to have their asses drilled by filthy strangers and decided to stay in the closet. These days most gay dudes I know want to find a nice guy to buy a house with and maybe adopt some kids, not paint themselves bright colours and waddle around the street in assless chaps.
So why is Billy so convinced that it's possible to change your orientation by witnessing gayness? The only reasonable conclusion is that seeing gay men depicted stirs something personally within him. He likes dicks, and wishes that he didn't. He finds it easier to blame some gay anime vampire for his feelings than to admit that they have always been there and he is deeply in the closet. Billy is an old-fashioned homophobe, and like many of them it's because he is scared that he will catch the Big Gay off them, whereas in fact the Big Gay was in him all along.
It's remarkably reminiscent of those male feminist types who turn out to be sex pests. They are drawn to feminism because they assume that all men are as rapey and misogynistic as they are and need a powerful punitive social structure to control their urges. Except that most men are not rapey, society doesn't make you rapey, you, the feminist, were rapey all along and you're just projecting it onto everyone else so that you can blame "society" or "rape culture" for your fondling of women's asses without their permission rather than your own flaws.