You shouldn't lie to your partner about your sexual orientation.
Bob is engaged to Debbie but is secretly gay.
Bob wants to be married and have a family.
Bob tells Debbie the truth, breaks up with her and marries a nice man and adopts kids with him.
Bob continues to make love to Debbie despite his misgivings and gets married to her.
Debbie is less hurt than she would've been and Bob finds happiness.
Debbie eventually finds out and is sad and divorces Bob and Bob becomes miserable.