I don't hear Christians saying "homosexuality is a lifestyle choice" as often as I used to. Instead the catch phrase that's been floating around for some time goes like this. A gay person will say, "Homosexuality isn't a choice." Then the Christian will respond, "But it is a choice whether you act upon it or not."
In my opinion this retort has a "So there. Nyah!" sound to it. It's just a comeback to parry an argument from the gay side. It's also a way of reducing someone's entire life's struggle over self-acceptance, social rejection, questions of faith and moral introspection down to a simplistic choice of "acting upon it or not." And making them feel judged for it.
Let's break down this exchange and see if we can put some thought into what ought to be the Christian's response. First off, when a gay or lesbian person says, "Homosexuality isn't a choice" or "I didn't choose to be gay," they are referring to a charge that Christians have been leveling at them for a long time, namely, that they do choose their homosexuality. So this person is really airing a grievance, and you should be thankful that he or she is willing to put it that nicely, considering that they are really trying to say that the church has been perpetuating lies about them
Here is a translation of what that gay person might actually mean: "I have been told by my church, or by Christians, that I don't really have involuntary same-sex attractions that I can't help. That I don't really fall in love with people of the same sex or am capable of experiencing real feelings of caring and spiritual bonding. Instead I've been told that I am actually a heterosexual who has perverted myself, or been perverted by molestation, so that I am now such a screwed up human being that I no longer know how to love naturally, but have become a morally and sexually warped person who lusts after unnatural things.
"I have looked at myself and asked myself whether this is true. For a long time I was afraid to introspect for fear of what I might find. Some people have killed themselves because they could not sort out the truth from the lies they have been told about themselves. But I've made it through all that and I know that this story about my so-called 'choice' doesn't ring true with the narrative of my life. Nevertheless, because this lie still hangs around conservative Christian circles, my family and friends think horrible things about me and this has damaged my relationship with them beyond repair.
"So now I am sharing this with you to disabuse you of hurting someone in your life who may be gay or lesbian: I did not choose to be gay. I should know since I'm the one who's experiencing it. Will you take what I have to say seriously?"
Now this is the part where you, as a Christian, might miss the entire context from which this gay person is coming. Instead you will be tempted to fight back with your zinger of a response so that you can go back and tell everyone in your church small group that you "witnessed" to a homosexual this week.
Don't do that. You must resist temptation. Primarily because you are probably slapping down a wounded soul, which is very un-Jesus-like. And secondly, when you answer the protest "Homosexuality isn't a choice!" with "But you can choose whether to act upon it or not!" you are actually conceding their point. You have shifted your ground without acknowledging that you have done so. You have redefined "choice" from "choosing to be homosexual" to "choosing whether to act upon one's [unchosen] homosexuality." By making this sleight-of-hand switch, you acknowledge that this homosexuality--which someone can choose to act upon or not--is in fact an unchosen condition.
A better approach would be to drop the superiority act. Instead clothe yourself with humility and try to come up with some honest words to say. Here's an example: "You know, I have to admit that for a long time I thought people simply chose to be homosexual, but now I'm not so sure it's as simple as that. I once knew someone who was ex-gay. Sometimes it seemed like she was making progress in overcoming her same-sex attractions and sometimes she was very discouraged. It got me thinking about whether it's realistic to expect people to completely overcome their sinful conditions. Okay, I know that you don't agree that homosexuality is sinful, but as a Christian that's the perspective I'm coming from, and I hope I'm not offending you by sharing where I'm at with this whole issue.
"Anyhow, back to my ex-gay friend, I remember that she used to say that even if she couldn't choose away her homosexuality, she could at least choose not to act upon it. You might not agree with her, but speaking for myself I had nothing but respect for her. As you were just saying, she probably didn't choose to be homosexual, but I admired the choice that she did struggle to make for herself every day."
Now if at this point you still get attacked by that gay person, fine. Take it on the chin. At least you can have a clear conscience that you've "witnessed" with kindness and courtesy. But my bet is that you will have succeeded in avoiding a fruitless argument and will find yourself involved in a much more meaningful conversation.