Like the old saying goes, if (s)he’ll cheat with you, (s)he’ll cheat on you.
Oh, without question. I dated a guy for a while who wanted to marry me, but I didn’t want to marry anyone at that point. I found out he was already married (!!!!!) and had two kids. (!!!!!) Not only that, it was his second wife, and he cheated with her before divorcing his first wife and son. A mutual friend clued me in.
I dropped him like a hot brick. I can’t abide a liar, and a cheat is always a liar.