How is it more humiliating to end a relationship because someone a cheat and a liar (and what you tell people is entirely up to you, it's your business but you can just say you discovered he's a serial cheat and had to end the relationship and this cancel any wedding plans) ....than to stay with & marry someone who's a serial cheat and liar.
Is that severe humiliation ok because it's only you two who know and you pretend it hadn't happened (and will you keep on pretending it's not happening when he returns to it at some point, which he probably will)?
It's not humiliating for you to end. relationship with a cheater and list before you enter into life changing legal, financial etc committment with him. It's you acting with self respect - the opposite of humiliating.
It'll be a bit of talk for a short while and then people will move on - they always do. And it's up to you how much you tell people, it's your private business, and it's your perpgativevto end a relationship at any time for anything you like.
As for 2 relationships that haven't worked out .... That's nothing.
Better to end a relationship with a serial cheater and liar than subject yourself to that as your life partner (which you will badly rue, I guarantee you, and will break down anyway unless you want to spend the rest of your days as a self deluded, std infected, stressed, sad door mat).
Do you come from a very conservative, traditional, judgemental background or something... That you put such stock in having only two relationships that didn't end up in marriage??!!