Homosexual relationship is the only sin that satisfies these conditions:
- It is natural and dispositional, in that one does not "prefer" to go homo, they are simply born this way. (EDIT: having a homosexual relationship is not a disposition, it is acting on said disposition. However, I would say that this is the natural and harmless way, as defended below)
- It is harmless to all parties. One may retaliate by mentioning the "harm to society, family structure etc." but I see these answers as rhetorical since the acceptance of homosexuality doesn't mean we must abolish traditional family structures, and the harm to society is purely subjective as the only rationale for this argument is the fact that homosexuality is a sin. If it were not, I don't believe any Muslim would be concerned about this issue. I haven't come across any secular research that portrays gay people as "a threat".
- It is not achieved through exploitation, unlike in the case of incest. Even though both parties are consenting, the possibility of grooming remains in an incest relationship. This is not applicable to a standard homosexual relationship. (and I doubt incest is dispositional, so there's that)
There are still some discussions about the word بَلۡ and its implications about the "true meaning" of the verse 7:81 and similar verses, but I will go with the orthodox opinion here, unless someone can convince me that that there are indeed wider discussions about the status of homosexual relationships in Islam.
One may make an argument by pointing out that "we all have our own tests". I can divide these "tests" to two categories:
The situation we're born into: being a child of a poor or rich parent, being blind, not having a body part etc. However, these tests are vastly different from homosexuality because you don't have to act against them. I find it hard to explain my position because it surpasses my knowledge of English, so I'll just do a comparision: The restriction on homosexual relationships is more similar to being forced to not use one of your legs, for example, even though it's perfectly healthy.
The "minor" tests, to be honest, that applies to all Muslims; stuff like not having sex before marriage (note that homosexuals don't have an 'endgame' in this aspect, since they can't get married), not eating pork and not drinking. Eating something other than bacon or abstaining from recreational drugs won't cause you to have psychological issues or greatly deteriorate your quality of life (provided you're willingly making these choices), but being forced to living a celibate life for no apparent reason (note that other orientations such as pedophilia or beastiality do harm living beings) will.
So why exactly is acting on a totally harmless trait one can't shake off considered a sin? And please don't get me wrong: I'm not asking these questions to ridicule Islam, I'm actually trying to symphatize with it.