I think it depends on the circumstances. I think many people have cheated on partners when they were teenagers, and were in relationships that lasted a few months, not years.
I wouldn't judge someone who cheated on their girlfriend/boyfriend when they were a teen, because, well, they were still kids, teenage relationships tend to FEEL very serious, but are actually very fleeting, they are working out how relationships work, and also most teenagers tend to be complete dicks! (myself included!). The vast majority of people grow out of that behaviour, and will even be ashamed for it when they are more mature.
As people get older, I think the level of disgust over cheating becomes directly proportional to the seriousness of the relationship (at least for me). If one of my friends in their thirties was seeing someone for a couple of months, the relationship was still pretty casual, etc..if they cheated on that person I would tell them it was a shitty thing to do, and to finish the relationship, but I wouldn't label them as absolute scumbags.
If someone has been together with another for, god, say more than a year, were living together, or talking about living together, and were a serious couple, then it's a very, very, very shitty thing to do.
The situation you describe though is pretty much off the scale. To cheat on your partner once you are engaged, or just before the wedding, or when you are married...to me that lands you in "irrevocable c*nt" territory.