A few days ago in an interview the Archbishop of Canterbury, Justin Welby, was unable to give a clear cut answer as to whether he thought homosexual acts were inherently sinful or not. See here https://www.theguardian.com/uk-news/2017/oct/02/justin-welby-unable-to-give-straight-answer-on-whether-gay-sex-is-sinful.
I'd like to help him out. If you mean by sinful spiritually unlawful then yes they are. An act against nature is an act against God and that is a sin. And these are acts against nature because, at the simplest level, they are using organs either designed or evolved (it makes little difference which word you choose in this case) for one specific purpose in a way that is quite contrary to that purpose. You may say homosexual acts are natural to a homosexual but that is irrelevant. Suppose I have a bad temper. Losing my temper is quite natural to me. That does not justify it. The bottom line (goodness, it's hard not to make double entendres with this subject) is that homosexual acts are a sin because they are a misuse of the creative energy of polarity for purely personal ends. Of course, much heterosexual activity also falls into that category and there the same rules apply. We would never have come to this position were it not for the inroads the sexual revolution has made into all aspects of human sexuality just as feminism, the female revolt against the male, would not have come about if first there had not been the male revolt against God. One thing inevitably leads to another. It's a slippery slope and it always leads downwards.
Actually I sympathise with the Archbishop. He is caught between two stools, that of the spiritual beliefs of the church he leads and that of what it means to be a good and decent person according to contemporary liberal ideology which basically amounts to treating everybody in exactly the same way regardless. But he should learn a lesson from the person he supposedly follows. I have quoted from the passage in the New Testament where Jesus confronts the crowd baying for the blood of the female adulterer before on this blog because it seems to capture so much truth in such a perfect and concise way. You will recall Jesus told the mob that he who was without sin should cast the first stone at the woman at which its members shamefacedly dispersed. Then he told the woman that he did not condemn her either and that she should go and sin no more. So her act clearly was a sin. That should help the Archbishop make up his mind. But at the same time she was not condemned, though one has to assume that she did repent and not sin any more for her forgiveness to be properly operative. Or, if she did sin, she at least recognised that it was indeed a sin and did not try to justify her act as pure.
There is the idea that in the context of a faithful, loving relationship homosexual acts might be acceptable to God. But this is to ignore the reality that they are a distortion, almost an inversion, of the energies of creation which energies are fundamental to the existence of this universe and so about as sacred as anything in our experience can be. To transgress the sacred is surely sinful, regardless of the excuse of love which word is misused anyway since the true motivation here is really desire. There is no sin in two homosexuals living together or even loving one another but their love must be pure which, in this context, means not expressed sexually. Undoubtedly such an attitude will be widely rejected nowadays and anyone holding it condemned as prejudiced, if not hateful, but that just shows how out of kilter our contemporary civilisation is with spiritual truth which it either rejects altogether or else sees in the light of the priorities of this world.
And that is the Archbishop's problem. He is spiritually weak. He is a liberal before he is a Christian so he sees acts as moral or immoral according to whether or not anyone is hurt by them in their outer worldly self, not according to whether they are in line with or against spiritual reality. He is more concerned with injuries to personal feelings than he is with those to the soul. He is right not to condemn the sinner. He is wrong not to condemn the sin. Indeed, by not condemning the sin he is condemning the sinner to spiritual error and its consequences and thus doing him greater harm in the long term. Surely the greater love is to lead people to the truth that sets them free.