You make a good point. THAT is what my Dad, did with us. My home growing up, was for friends. My friends loved it and my dad/Mom. They even thought of them as "their" Mom and Dad too.
And then that way, my parents/Dad could gauge our friends and situations AND problems.... and we always had 2-way conversations... openly. COMMUNICATION is key... and respecting the Teen as well.
Teach him about sex/birth control.
If you don't 'allow' your Step-Son/Girlfriend some hospitality, they will do it elsewhere. Yes. Most teens do. But have 'rules.' They are still kids.
The human brain is not even fully developed until 26 years old.
Have Dad/You talk to him... about things, expectations, responsibility etc.. WITH him. Not talking 'down' to him. 2-way talks.
He is still a teen... and she is too. Parents of girls are often more protective... so you can get to know them TOO. Her parents. Do you even know them????
Designate curfews, situations, that they call you anytime their plans changes... if they are late, in trouble etc.. They are kids. But always be open... honest, respectful... and if he does not behave responsibly, then you can cut back on how often he goes out, etc.
It is a 'privilege' to be independent with a girlfriend.
ALSO teach him about how to treat girls, and to KNOW if a girlfriend is not appropriate either. Some girls are aggressive/manipulative etc. But that works for boys too.
Since he has a girlfriend, I would with Dad, sit down and discuss "life."
all the best,
Susan