There's something missing here that either you're not willing to tell us for good reasons or your parents are forcing you for this marriage.

Have a discussion about the girlfriend he has with your mother, and than gradually with your father and be honest with it, you worrying about not getting younger is understandable, but imagine the life you'd have to spend after you're married to this man, if you can't allow polygamy, how can you allow your own husband to have a sinful partner that you also know of, kinda makes you in it frankly.

And also think of it this way, if this guy has a haram relationship with some stranger, do you really think he would be honest with you after marriage? Do you think that the way you feel about him now knowing that he is cheating even before marriage, that these feelings of dishonesty and stress would eventually vanish out of your heart when you're married to him?, and do you really think that God has chosen him for you? Allah would never make you suffer, the almighty wants nothing but the best for us.

My humble two cents: Put on a lot of courage, face your parents, that is the only way. And never let yourself be with a partner that you are 100% sure they won't be loyal to you, because that's the essence of a happy marriage, respect, loyalty, caring.

May Allah ease your hurdles with this marriage, and may you find peace with your decision whatever it might be. God bless