Is Homosexuality a Sin?

Yes and no, since the answer depends on the context. There are two different kinds of homosexualities: natural and unnatural. Regarding the natural, this kind is a disease one is born with and is rare. For there may be someone who, from birth, has homosexual inclination or a male being or feeling effeminate or a female being or feeling masculine. The disease itself is not a sin, for the one who has it is not culpable for it, though acting with the disease is sin. Now, regarding the unnatural, this is anyone who is not born with the natural disease of homosexuality. So any action or willful inclination is a sin, for those people have chosen it. 

Participating in homosexuality is a sin since it is against natural law. We see the men of Sodom (Gen. 13) being punished with fire for their homosexuality, and so it will be for those who do not repent of their homosexuality. We see Paul rebuke those who are doing what is unnatural (Rom. 1), whereof he is speaking of homosexuality. So unless a homosexual repents and puts away his unnatural and wicked sexual desires, he is in mortal sin.

Prayer for the homosexuals:

Pater noster, qui es in caelis. Sanctificetur nomen Tuum. Adveniat regnum Tuum. Fiat voluntas Tua, sicut in caelo et in terra. Panem nostrum quotidianum da nobis hodie. Et dimitte nobis debita nostra, sicut et nos dimittimus debitoribus nostris. Et ne nos inducas in tentationem, sed libera nos a malo.

Ave Maria, gratia plena, Dominus tecum. Benedicta tu in mulieribus, et benedictus fruictus ventris tui Jesus. Sancta Maria, mater Dei, ora pro nobis pecatoribus, nunc et in hora mortis nostrae.

Lord, come to the help of those struggling with homosexuality that they may see Thee, O good and loving God. Have mercy on them and show Thyself to them. Amen.