The original question seems to be “If a person considers homosexuality to be a choice of free-will, shouldn’t they also consider heterosexuality to be a choice of free-will?”
First, as another poster noted, there should be a distinction between homosexual orientation and homosexual acts. Though there are undoubtedly some who consider even the thought of desiring the same sex to be sinful based on Matthew 5:28 and the various prohibitions from the Pentateuch, I doubt most Christians who see homosexuality as a sin believe this, and even thouse that do consider it a lesser calibre of sin that the actual homosexual act.
There is no doubt a person chooses to indulge in sexual acts, just as there are plenty of folks who indulge in celibacy. Theologians argue that our status as rational creatures allows us free choice in the matter of what we do with our sexuality; in short, in contrast to the instinct of animals, we are not slaves to our biology. The answer then to the original question is yes.
This may strike us as unfair because the presence of a condition (homosexual orientation) that makes a sinful act (homosexual relations) more tempting (i.e. less a product of free-will) should mitigate the consequences (damnation). For example, though killing in general is prohibited by the 5th/6th commandment, killing in self defense should not bear the same penalty as killing for sport. The theological answer, I think, is that:
(1) God has decided that some are born with heavier personal burdens for purposes we cannot understand. Applying this logic, people born with homosexual orientation have a personal cross to bear, much like someone born with Cerebral Palsy or a predisposition to alcoholism (I am NOT stating that these lead a person to sin, merely illustrating that even theology acknowledges we don’t all start in the same position). In order to lead a Christian life, a person born with homosexual orientation has an additional–but not unbearable–burden of celibacy. I’ve always thought this was a cop-out answer, but there it is,
(2) the conflict is completely internalized, and so completely within the control of the person (defeating the killing-in-self-defense analogy).
My personal opinion is that religious persons, while perhaps conceding that no person is harmed by consentual homosexual acts, they do say the acts are a sin against God’s law. Such an appeal has always made me suspicious, as an appeal to an approved canonical interpretation of God’s law is exactly what Christ argued against in his debates with the Pharisees. Consequentially, more recent arguments point to the damage to society cause by the tolerance and (what many see as) acceptance of the homosexual lifestyle, e.g. it will lead to an eventual breakdown of the family structure, people will gleefully have sex with horses, <insert your favorite bigoted remark here>. The kernel of that debate–the effect a tolerant attitude toward homosexuality would have on social structures–is worth considering without the usual biased vitriol, and may be the only route toward resolving the lingering social problem of intolerance toward gays.