/I would have to wonder where I went wrong/
If your kid abuses drugs, kills someone, picks up prostitutes, runs away from home, bullies another kid to oblivion, drops out of school, abuses his girlfriend, or commits suicide, THEN you can wonder where you went wrong. Otherwise, shut the heck up.
/And if he did, I would forbid him to ever see this person again,/
Because that worked out so well for Romeo and Juliet.
/remind him of exactly how and why this behavior is sinful,/
Causing him to lose any trust he may have previously had in your intelligence and competence as a parent.
/take him to a priest for confession,/
I won't take the bait. This is just too easy.
/go to confession myself for my failure in allowing it happen in the first place,/
"Allowing" it to happen? How the heck do you cause your son to fall in love with a boy? You don't MAKE people fall in love with each other, they just DO. Give me a break.
/and then have both of us do serious penance./
*Headdesk* You're a horrible parent.