I wouldn't tell someone to run just because they were in a new relationship.
I can say, though, that after my experiences, I would definitely run fast and hard and save myself from unnecessary heartache if the WS was unremorseful.
But, you have to look at the whole picture. If the WS is truly remorseful, giving transparency, complete NC with the AP, etc., I would NOT recommend D. But if a WS who also happens to be in a new relationship, when he or she should still be in the honeymoon phase of the relationship is already cheating, not truly remorseful, hasn't been transparent, and continues contact with AP, or suspected contact with AP, AND is a serial cheater from previous relationships, I would definitely advise to quit while ahead.
Most of us have lots of experience and hindsight on our side.
It makes me MAD, I mean boiling MAD when I see the emotional abuse of not only the A, but the lieing, projecting, gaslighting, and blameshifting that some of the BS on here suffer. If I could prevent even 1 person from experiencing years of that shit, I would do my best. If it is a new relationship, and there is no true R, hell yes I would want that BS NOT to go through what some of us endured years of. Take a look at Nature_Girl and SBBs stories. They are survivors, but bet money they would rather have not had to endure half the abuse their Xs put them through.
I truly doubt most of the people here would advise even a new relationship to cut and run if the WS was showing true R. Some, maybe, but most, highly unlikely.
Perhaps, the person advising to cut and run saw signs that you didn't see yet, of the train wreck waiting to happen. Maybe they didn't know how to adequately express this sentiment to you.
I cannot tell you how many times I have just kept my mouth shut, because there was no way to nicely tell someone that they were sticking their head in the sand.
Nobody deserves to be cheated on. If the WS, regardless of how short or long the relationship was going on, is NOT truly R and NOT putting in the effort, then it's time to walk. No it's time to run. But especially if the relationship is short lived, save yourself the heartache.
Love is supposed to be just that LOVE. It isn't supposed to make you cry, it isn't supposed to devastate you. It is supposed to uplift you, and make you each strive to be better for each other.
Love isn't abusive, sneaky, or hurtful.
I put up with almost 3 years of that asswhole treating me like shit, or worse complete indifference. That's 3 years of my life. But we were together almost 10 years. I promise you, if he had treated me even half as bad in the beginning, I would have left his ass 10 years ago after the first sign of cheating. If he had been super fantastic, then cheated, but then remorseful, that's tougher. I was younger and didn't have as much invested. I might have left and I might not have. I do know this from watching a lot of relationships end from infidelity. Most of the WS treated their spouses like gold in the beginning, and it never got better, only worse.
Only you know your whole relationship. Only you can determine if he is worth the hard work of R. Because if you've taken a gander over to the Reconciliation forum, it's no walk in the park.