There are basically four scriptural passages cited in the Catechism section 2357 to justify the Church's anti-gay stance. They are as follows:
1 Corinthians 6:10
1 Timothy 1:10
These passages have all have two important things in common. One, you can notice just by looking at them, is that none of them are from the Gospels. Very rarely do people use the actual words of Jesus, the ultimate authority in our Church, for justification of this viewpoint. Two, none of these passages is primarily about homosexuality. They're all talking about something else, and the homosexuality is incidental.
Let's start at the beginning with Genesis 19:1-29. All you really need to know about this passage is (a) it takes place in the city of Sodom and (b) it's about an attempted gang rape. As any competent mental health professional will tell confirm, sexual violence is motivated not by sexual attraction but by a desire to exert power and control over another person. Therefore, both the gender of the victim and the orientation of the perpetrator are irrelevant. Moreover, if all biblical rape stories were used as justification to condemn consentual relationships, heterosexual marriage would also be condemned.
Incidentally, the way that the intended victims of this assault attempt to escape is to send out a servant girl (forcibly, no doubt) and say, essentially, "Here, take her instead and leave us alone!" In other words, they sacrifice someone more vulnerable in order to try to save themselves. Have you ever wondered what is the opposite of what Jesus would do? This is it, right here.
Depending on the translation, this story is alluded to in both the above passages from 1 Corinthians and 1 Timothy. I'll plan on dealing with them tomorrow.