Some things seem to repeat
I must agree with Wildcat on this one,, I tend to go with the once a cheater, always a cheater. Not saying people can't change, but does't really seem to be the case here.
I believe that you can only truly love one person at a time,, sure you can love more, but not truly love more than one other. If you are already having doubts of marriage, and both have had affairs, then the baggage is to dear before the marriage even begins.
A relationship takes two people giving, yet it only requires one to end it. Seems strange that people feel obligated to stay in something they know is not right. It is very difficult for a marriage to work and in your situation, it appears than instead of working on the relationship, you both find alternate solutions to get you though the rough times. In a marriage, one would hope that is your partner that you go to (or come to you) when trouble arises,, well before an affair occurs.
Just the same, I wish you the very best of luck. I have read many of your responses and enjoy your inputs to their posts.
Good luck!