Go for the double bind.
For example, if someone says they believe homosexuality is genetic, you take that inch, and use it against them.
So if homosexuality is genetic, and given that genes work in one of two ways (they either mutate like a genetic disease, or are inherited like eye color), you can then ask, well how do people inherent the gay gene?
Think about it, shouldn't the gay gene be self limiting each generation? If that were the case, gay people would be in similar numbers to people with genetic diseases, but they're not. There are tonnes of them, so how is this gene surviving and spreading through each successive generation?
Well, the only logical conclusion, is that the gay gene has existed, but thanks to societies of the past being homophobic, and giving people a choice of expressing their sexuality or being burned alive, gay people lied, fucked the opposite gender and passed their genes on. Ironically this means that the only reason we have a thriving community of homosexuals today is because of the homophobic communities of the past.
I mean think about it, had past societies accepted Tim and Bob fucking, they would have been genetic dead ends. Instead they repressed their true sexual identities, and today get to enjoy a thriving subculture.
At this point you can just twist the knife however you want, you can say that if people were loving and accepting of people then gay people wouldn't exist, so homophobia, ironically, was the least homophobic option for the species. You can flip it the other way and go if society really wanted to get rid of gay people, they would have just let them fuck themselves out of a subculture, etc. Either they stick by their original belief (there is a gay gene but now they believe in a much more nihilistic universe), or they backpedal on the gay gene, at which point you hit them with the 'conversion therapy must work' then. Because if it's not genetic, it's all in your head.