It’s a common catch-cry of the lesser spotted homophobe: homosexuality just isn’t natural because babies.
And it’s no good trying to convince them with the many, many documented instances of homosexuality in animal species, because we’re dealing with bigoted assholes who don’t even really believe what they say; their goal is really just to bully everyone into following their personal lifestyle choices by any means necessary.
But just for fun, next time you’re confronted with the “it’s not natural” argument, consider dropping this into the conversation:
Scientists have been studying the males of a particular species of Amazonian frog. During that time of the season, the males just get so darn vigorous that often they accidentally drown the female they’re all trying to hump.
At which point they give her a good ol’ squeeze to get her eggs out and jizz on them, thus completing the Circle of Necrophiliac Life.
That’s fucking natural, that is. And you can’t argue with Nature.