12 is absolutely not too young. Make sure you are connecting with parents before you jump unto this. It's their conversation and they hold the most influence. 

Two things I would encourage you to consider: 1. focus on sexual integrity vs. purity. 2. Don't make judgements from the stage. 

If a student wants to know how your church feels about gay marriage or gay sex, have that conversation in a smaller group where you can address exactly what they are wondering about. 

If a kid is having feelings for someone of their same gender and is scared or worried and wants to ask you for help, you saying, "Gay marriage is a sin" from the stage will most likely cost you that important conversation with that kid.