The homosexual inclination is a disordered inclination. While it is obvious that homosexual sexual acts are wrong, it is also disordered to carry on a romantic relationship with a person of the same sex. For two reasons - one simply because it is the encouragement of a disordered inclination. And second because it is clear that romantic relationships exist for the purpose of leading to marriage, sexual union, and procreation (or at least the possibility of such). To have a romantic relationship that could not possibly end with any of those things is disordered in itself.
See what the Catechism says about this (emphasis in red added by me):
2359 Homosexual persons are called to chastity. By the virtues of self-mastery that teach them inner freedom, at times by the support of disinterested friendship, by prayer and sacramental grace, they can and should gradually and resolutely approach Christian perfection.
(
beliefnet.com/Faiths/Catholic/2002/01/The-Roman-Catholic-Churchs-Official-Teaching-On-Homosexuality.aspx#ixzz1luBrcYFO)
Note that the Catechism bothers to put the adjective “disinterested” in front of the word “friendship.” Without even saying romantic relationships were ok, they could have left the word “friendship” alone. But they made the point to add the word “disinterested,” making the point that romantic relationships are NOT included in the list of ways homosexually inclined people can approach Christian perfection.