Update to correct factual errors:
When I teach seminarians about SSA, I emphasize that the love between two men who engage in same sex activity may in fact be quite genuine. The problem is that the same sex activity undermines the love.
Two men are able to love each other deeply and genuinely and with great emotion. This is a good thing established by God. It is a great thing to recover this.
There is so much suspicion these days that men find it hard to show this kind of affection for each other. I think that is why J.K. Rowling said she always thought Dumbledore was "gay" because of his obviously deep love for Grindelwald. I would say that the fact of the love does not mean that D. was "gay" or that he even had same sex attraction. Now, I haven't read the books in which this is discussed, so maybe I'm off base.
At any rate, St. Leo the Great says in his sermon on the Beatitudes, "Even the most intimate bonds of friendship and the closest affinity of minds cannot truly lay claim to this peace if they are not in agreement 'with the will of God.' He's talking about the beatitude "Blessed are the Peacemakers, for they shall inherit the earth." How do we know the will of God? Ask the Church. She knows. If she in her wisdom says that same sex activity is contrary to the good of the persons and society, then we know that our love cannot be expressed that way, whether it "feels" like "love" or not. We've got to learn to distinguish between affection for persons and sexual attraction.
Or, we can do what Johanan, son of Kareah did and ignore the words of the prophet Jeremiah and flee to Egypt with all the people of Judah (including Jeremiah and Baruch). Look where that got them!
Same is true of fornication, adultery, etc. Whatever genuine love might be in these relationships, it is obliterated by the action that is contrary to God.
I know I harp on this, but the best analysis of all of this is in Love and Responsibility, by a Polish guy named Wojtyla.