You are in denial because you are trying to protect your engagement and the thoughts of everyone else around you. That is why you are fraudulently posting untruths on social media, so everyone acts like you do, as if nothing has happened.
You had this fairy tale view of your relationship and your plans to marry and now it has all been blown to schit. Parading around like he never had an intimate relationship by sleeping with another woman while he was sleeping with you is a protection mechanism which allows you not to face the reality that he really did cheat on you and betrayed you in the worst way.
You need not tell anyone what truly happened. But, you would be so foolish to remain engaged to this guy because he really does not love you as much as you had hoped. You are living in a fantasy, not reality. If you decide to ignore his cheating and marry him anyway, you are a fool because he will cheat again, he will.
I would give his ring back and tell my friends that "something happened to change your mind" or you can be truthful and tell everyone he cheated on you and you broke it off as that would be totally understandable. No one is going to think badly of you, they will simply see him for who he really is, an unfaithful loser.
I suppose you can go on your merry way and pretend his affair away. But, YOU will always know that his love for you was not strong enough to keep him faithful to you.
He is no prize. He ACTIVELY sought out another woman to have sex with. He went on line, fished in the pond of single women, hooked one in and had an affair. He lied to her. He lied to you. He lied as he snuck around to meet up with her for sex. He lied to her about being engaged to be married to you. He cheated her out of an authentic relationship. He cheated you out of a real engagement. The only thing he is engaged in is his selfish wants and needs, regardless of who he has hurt. He had sex with her. He was intimate with her mind and body...then he came to you after sex with her, lying and scheming every single day until he got caught. You would be a total fool to marry him, and if you do, you get exactly who he is...a cheater and a liar.