Some people feel more attracted to members of their own sex than to the opposite sex. In itself, this is not a sin. For Catholics, marriage is a bond between a man and a woman. A ‘homosexual marriage’ cannot be fruitful in the same way as a marriage between a man and a woman, who share their love in their sexual relationship, from which children can be born naturally.
At the same time we should not judge other people harshly. There is a big difference between who and how people are, and what they do. It is a very important Christian command to treat everyone with respect and sensitivity.