The Church teaches only what God has revealed through nature and the Scriptures. It is apparent and objectively obvious that we are not designed for sexual activity with people of the same sex. Such act cannot unite two persons and it cannot procreate a third person — and union and procreation is the double-purpose of human sexuality. All sexual acts perverted from this natural end are intrinsically evil and abusive.
People are attracted to becoming Catholic because they love the truth, goodness, and beauty that is Christ reflected in His Body, the Church. Man-woman marriage is not something the Church makes up, or simply likes, but it is the natural outgrowth of human nature where a man and woman become one flesh. Same-sex marriage is like referring to a circular square; it is a contradiction of human nature that makes no sense.
Therefore, a person (or persons) that has a same-sex attraction, who would like to join the Church should work with a good priest. It makes sense that they would have to renounce any lifestyle that is objectively sinful as well as any false notions of marriage they have; but changing one's life may be a process.
A good priest would be able to help them overcome vice and live Godly, chaste lives, if they are serious in their desire to please God.