i) Ethical debates typically involve a search for common ground. One method is to use examples of something morally obligatory, permissible, or impermissible which both sides agree on.
When Christians debate atheists, this can be elusive. Secular ethics is an elevator. It has no moral floor. An atheist can instantly lower the moral floor at the touch of a bottom. As secular ethics becomes increasingly and systematically anti-Christian, it's hard to come up with examples that an atheist will balk at.
ii) Secular ethics generates a dilemma. On the one hand, an atheist demands the freedom to do whatever he wants to do. He rankles at anyone imposing on him.
On the other hand, he wants everyone to respect his freedom. So he imposes on them. No one has a right to obligate him, but everyone is obligated to respect his rights. That's the conundrum.
iii) Homosexual and transgender apologists resent it when Christian ethicists compare their position to something like pedophilia. Here's how one ethicist frames the comparison:
It's not true that there's never any room for such comparisons. It might be that certain standards for loosening laws will (by the same standard) require loosening them in other ways. If the pedophilic sexual orientation is out of someone's control, and that's the reason we reject arguments against homosexuality, then by the same standard we reject arguments against pedophilia.
Many issues about pedophilia as a sexual orientation are analogous. I haven't seen any argument from anyone that pedophilia is not a sexual orientation. It's usually dismissed out of hand. I also haven't seen any strong arguments that the formation of sexual orientation is different in the two cases. If the only argument given for accepting homosexuality as perfectly legitimate is that it's a sexual orientation that one can't control, then more work needs to be done to exclude pedophilia as a sexual orientation as a perfectly legitimate way to be.
iv) From what I've read, homosexual and transgender apologists usually invoke consent as the differential factor. Let's consider that. From a Christian perspective, I think consent is often a morally relevant factor.
However, that's not a universal principle. Consent doesn't ipso facto make a transaction morally permissible. Conversely, lack of consent doesn't ipso facto make a transaction morally impermissible.
v) For instance, if someone is mentally ill, they may be in no condition to give informed consent for treatment. Suppose psychotropic drugs can restore their sanity. In that situation, I don't think it would be wrong if, say, their brother authorized medication against their will. If the patient is not in his right mind, you are protecting him from himself or others–just as you might take the car keys away from a drunken friend and drive him to your own house until he dries out. Once he sobers up, you return the car keys. It might then be a question of whether maintenance should be a matter of consent.
vi) In a pilot episode ("Eyes") of Night Gallery, Joan Crawford plays a wealthy heiress blind from birth. She procures an eye-donor who's hard up for money. Of course, the eye transplant will leave him blind.
He consents to the operation. Still, the viewer is supposed to regard the woman as a villain. She's exploiting his financial desperation. She's harming him to benefit herself. The fact that the transaction is consensual doesn't make harvesting his eyes morally permissible. Even if somebody consents to be harmed, that doesn't automatically give you the right to harm them. Indeed, that may be taking advantage of his weakness.
In an episode ("Queen of Heaven") of I, Claudius, Tiberius has designs on the nubile daughter of belly dancer. To spare her daughter, the mother (Lollia) offers to take her daughter's place. She thereby subjects herself to his depravities. Later, she commits suicide.
Even though she volunteered to take her daughter's place, the fact that the transaction was consensual doesn't make it morally permissible.
It's easy to come up with other examples. Unbelievers who support homosexuality but oppose pedophilia need a more discriminating criterion than consent to warrant their dichotomy.