I think it also helps to think of it in terms of sex so they can keep holding onto their beliefs. If they start with the idea that sex is dirty and wrong, and they generally do despite it being as essential to our existence as waste elimination which is also "dirty" because I suppose it reminds them that we're animals, and then add to that idea having sex with a gender they personally wouldn't (unless your reading of the situation is correct, and they feel guilt for whatever reason) then they can dismiss it as an act or "practice" of someone violating "nature"' or god's laws. If they have to think of it in terms of love, of one person loving another who incidentally shares their gender, it's a lot harder to judge or condemn that. Not that they wouldn't, but it wouldn't be as easy.
To reduce an entire relationship to one aspect, sex, is insulting not only to homosexual relationships but to all relationships (especially their own). I'd find it a lot easier to respect their arguments that two men or women shouldn't be together because they can't make a child the traditional way if they extended that to infertile couples or couples that meet and fall in love after their procreating days are behind them (I'd think it was awful, but I could at least respect their consistency). But they don't, so in the end it's obviously not about that. Luckily things are getting better, and I think one day people will be as embarrassed of the anti-gay movement as they are to see pictures of white students shouting and spitting at black students in the '60s or signs that read "Whites Only".