It's because homosexuality can be thought of in more than one way. Therefore someone can read it in another way than the question was intended. When you step back to look at the bigger picture and consider the ways that "homosexuality" can be defined as an identity, an inclination, a lifestyle, a sex act, etc., it's far from a very simple question. Hence all the angst over answering it.
Imagine, if you will, that I brought Priceless home and we made sweet love until it got light outside. That would be homosexual. Would it make me "a" homosexual? Not unless he's incredibly talented. The first one is a choice.
There are also an unknown but high proportion of people who start out some shade of bi who end up "choosing" to identify exclusively one way or the other depending on their circumstances. Then it's a choice.
Here's a good
essay to read before you tell a gay person that they don't have a choice:
"Should queer people only receive equal protection under the law if we can prove that we're born this way and can't help it? Or do we perhaps deserve equal dignity simply because there's nothing wrong with being queer, and because how we live doesn't violate anyone else's rights?"