In light of the Proposition 8 decision and of recent media accounts of fluidity of female sexuality, people are discussing the significance of the question of whether people choose their sexual orientation. I will ignore the ignorant conflation of fluidity with choice and will instead give my views on whether it should matter and whether it does matter.
For multiple reasons, it should not matter whether people choose their sexual orientation. It is undeniably true that people can choose to convert to a different religion or to abandon religion altogether, yet no one claims that that freedom to convert makes people having politically unpopular views on religion fair game for discrimination. Also, in libertopia, people would have the freedom to choose their sexual orientation insofar as they could do so, unless in doing so they somehow initiated force or fraud against someone else.
Nonetheless, "should not" does not mean "does not." While libertopia is a long way off, the 14th Amendment is the law now, and immutability of a characteristic has a lot to do with the level of scrutiny that courts will give to discrimination on the basis of that characteristic. We must also consider political reality; if we can use that reality to our advantage without committing intellectual dishonesty, why should we not?