It is irrational, unnatural and therefore immoral.
Homosexual behaviour itself has no rational justification. SSM isn’t even on the table.
Homosexuality necessarily requires a subordination of human reason to the passions, even contrary to nature and reason. If someone has a genetically based inclination to eat dirt, it would not make eating dirt rational or recommending it as a food substitute moral. It would, however, call for our sympathy insomuch as the person cannot help it. We should not hold them in, e.g., contempt; however, we must insist that they cannot eat dirt. Dirt is not food even if we are starving. Moreover, to try to, e.g., force the government to declare that dirt is a natural and even healthy meal would constitute a crime against humanity and a serious betrayal of duty and responsibility. Dirt is not equal to food and food is not dirt.
People afflicted with same-sex attraction are called like anyone else to control their passions as, e.g., people who are born with a genetic inclination to addiction (as many people are especially when their parents were addicts). Exactly because they are more liable to become addicted to certain substances, it is all the more reason for them not and never to consume them. Homosexuality is little different, except that unlike certain substances like, e.g., alcohol consumption, it is aways an abuse and contrary to nature. There is no modest indulgence in a homosexual act.
Like anything, culpability can be reduced by ignorance such as lying propaganda that wickedly and cruelly presents homosexual acts as somehow being natural. Notwithstanding, we all know thanks to the natural law and conscience that it is not natural. We learn fairly quickly what our body parts are for and what they are not for. We learn what is food and what is not food.
The sane and sober person says that eating with a fork is perfectly natural and rational; but to try to eat the fork would be insane and unnatural. Every pro-homosexualist argument would ultimately have us believe that eating with a fork is irrational and unnatural, and that eating the fork is perfectly rational and natural.