Greg got a question on his Facebook page about how to respond to this cartoon about homosexuality.
There are some problems in the reasoning. First, the question of whether homosexuality is chosen is a red herring. True, Christians do bring it up, but it's really beside the point how someone becomes a homosexual. What is really at question in these discussions is the morality of homosexuality and if society should endorse it. When people argue that it's biological, they are making the unstated assumption that what "is" "should be." They're making the leap from "is" to "ought" and that's a logical fallacy.
That's the second issue here, and really the heart of it. Get past the red herring and go the the moral issue, which is really the central one. Because something is natural doesn't mean it is moral or should be accepted. And you can give many counter-examples to illustrate that. If someone were naturally prone to violence, we don't accept it. Because some people seem naturally inclined to pedophilia, we certainly don't accept it. By giving counter-examples, you aren't assuming homosexuality is like these things - that requires further argument. What you are trying to show them is that the argument offers that homosexuality is natural is not enough to claim it is also moral. And that's really where you have to move the conversation, though you can't do that until they are willing to give up the logical fallacy inherent in this leap from is to ought. And honestly, that's really where most of the public dialog on this topic stalls because this unstated assumption is rarely ever made explicit and examined.
One caution about counter-examples. Sometimes people are willing to accept something absurd to preserve their point of view. They'd rather be absurd than wrong, so don't be surprised if someone doesn't see something wrong in your counter-example. But in that case you've found someone who isn't willing to be fair - at least at that moment. And all you can do then is leave the stone in his shoe and hope that he thinks about it again in a more honest moment.
The other thing is that since the sexual revolution, our society has generally bought the idea that sexuality (not just homosexuality) should be expressed as a person desires and is an amoral issue - not moral. So this assumption may also be lingering unstated. Counter-examples can also bring this out because there are still a number of sexual behaviors that are considered repugnant and there's nothing in principle to make some moral and others amoral. So be alert to that.