I don’t see any reason why a guy will engage in adultery with a girl he can’t marry or take as wife.

When you engage in Adultery with who is not your wife, you are killing yourself unknowingly, bringing misfortune, setback, death and so on upon your life.
Even in Exo 20vs14 it says
New International Version (NIV)
14 “You shall not commit adultery.

But we all have disobey this commandment and follow our own direction.

Stop having Sex with her if she can’t be your wife, let her go if she not your wife.