If you like being a cuckold, you will keep dating her.
If you don't like what is happening, then she will become your ex-girlfriend very quickly.
You are very young, and cuckold emotions are INCREDIBLY INTENSE, and only some guys can handle them.
Do more of what feels good, do less of yucky things.
You are saying "yucky" to being cuckolded by this other guy,
SO FIND ANOTHER GIRLFRIEND who will appreciate you and want to be with you to the exclusion of all others.
It is possible to fall in love with lots of girls in you lifetime, so if this current girlfriend is stirring you up with her "adultery", then the solution is simple - MOVE OUT OF HER LIFE QUICKLY WITH AS LITTLE FUSS AS YOU CAN.
Shake the guy's hand and say, "she is all yours", and thank your girlfriend for the time she gave you. Don't ever criticise your g/f to other guys, just say that you are pleased she is happy with him, and you are NOW FREE to find new love again.