Your son has just brought his new girlfriend into your house and went inside his room and closed the door. Is that okay with you?

Parenting 101
Keep your children safe and healthy.
Teach your children (who are sexual beings) to respect themselves and others.
Teach them about reproduction and how *not* to reproduce
Teach them about STDS
Keep them supplied with condoms because, regardless of how you feel, sexual beings will have sex.

If I have done all of the above, I would try to mind my own business.