T.N.
Gasp! A post puberty teenage boy talking about sex?!
Please buy him some condoms. Please show him how to treat a girl with respect.
You will probably not be able to stop him. But you CAN teach him self respect, how to keep himself safe, and that GIRLS ARE TO BE TREATED WITH THE UTMOST reverence at ALL TIMES!
Better still, talk to him non threateningly about the girl. She's his GF? Have her over for dinner. Take them on a date. Get to know her.
Sexuality is as much a part of him now as his grades, his health, etc. Why pretend it's not there or make him feel like he's BAD in some way for having hormones?
Least, that's what I've done with my teenage boys. So far so good.
:)