Gwyn - I'll post what may be a controversial opinion here (and note, this is coming from someone who successfully repaired her marriage - but we had 20 years and 3 kids invested).
IMHO - you're right. You don't have a lot of years invested, don't have kids with him. It wasn't a one night stand or a brief fling, but an ongoing, 1 1/2 years of lying. Your R wasn't old or stale. This says something pretty serious about him, his character, and his ability to be in a truly committed R.
The more typical story here is of a marriage that was basically good, then due to the WASs depression and/or external pressures and/or gradually drifting into a rut in the R, the vulnerable WAS is easy pickings for someone who pays attention to them or makes them feel important again. Sad, yes, weak, yes, but understandable and often forgiveable. And when there are children involved, or a shared history of many happy years together, it makes it worthwhile to fight to get through the pain.
But someone who cheats on you this early in the R most likely has something else going on (especially a grown adult male - it's not like he was a 20 year old who feared he might be missing out on something!). And I think you are perfectly within your rights to decide that he is not a good bet for a long and happy future together. Especially if he hasn't figured out why this happened.
Is he sad to lose you, but doesn't want to confront why he did this? Is he getting treatment for his depression and/or substance addiction? Has he ever cheated on a past girlfriend or ex-wife? (Don't take his word for it - go right to the sources!).
I'm betting, in a case like this that happened so early in the R, there were other red flags you chose to ignore. What are they?
Look - if YOU want to save this marriage, because you feel there is more good than bad, and you aren't planning on having children with him - then do whatever you feel like doing. If you feel that this is just too big a character flaw to overlook, and not what you signed up for - (you wanted, after all, a man who would love and cherish you and be your rock of support in bad times, right? ) - then you have every right to decide that this is just not the person you want to spend your life with.
