I-S: People are raised to be Christians, so to some degree, it is who they are. Yet it is possible to decide that you will no longer be a Christian. On the other hand, we don't get to choose our sexual orientation, and we can't just decide to change it. Most of us are heterosexual, and we never faced a decision to become heterosexual. It just happened that way, and we have no choice in the matter.
VR: I think there is a lot that is problematic about this statement.
First of all, many atheists (Dawkins is an example, but there are others) deny the existence of free will, but this contrast won't float unless we do.
Second, traditional Christians don't typically claim that homosexual orientation is a sin. They often claim that homosexual acts are sinful. So, if your orientation inclines you toward members of the same sex, they may say you cannot engage in morally justified sex. Christians have also traditionally claimed that heterosexual acts by persons not married to one another are also sinful. Since not everyone is in a position to enter a marriage, that means that those persons are also barred from sexual activity.
The critic of Christian opponents of homosexual activity, therefore, are assuming the view that persons have a moral right to fulfill their sexual desires in accordance with their orientation. But, thus stated, this argument comes into problems. We have to then get clear on what constitutes an "orientation." Richard Carrier has recently "come out" as polyamorous and has said that this is his orientation. And if we accept his claim, then what is next? Sadomasochism? Pedophilia? At the very least with the last category, I think most of us would be inclined to say that people with that orientation are obligated to remain celibate rather than act out this orientation.
But we can surely choose what we do about our sexual orientation, whether to pursue sexual activity, or not.
Atheists, often in response to Pascal's wager, tell us that they couldn't will themselves to be Christians if they wanted to. I don't think I could turn myself into an atheist by force of will, either.
Hence, I don't think the contrast I-S wants to draw will work, at least not the way he wants to draw it.