Hi Rachel,
If he’s doing this to you… and you stopped… chances are he will cheat on his girlfriend with someone else. Who he dares bring back to HER condo!
On some level the girlfriend knows. Or will one day confront reality with someone who would tell her.
As for you, I wouldn’t be his dirty little secret. Tell him it’s all or nothing with you, and now that you know he has this character flaw, it is now and forever nothing.
And, bonus, IF the girlfriend asks about you and him, tell her the truth. All of it. Don’t tacitly agree to protect him. That assumption right there is arrogant of him. (Half of all women WOULD tell!)
Best,
Inky