This is really dependent on your personal opinion of what makes a person "decent", and that will be defined by your personal morality. Other people will have different opinions because they don't share your personal set of values and morals.
Personally I think anyone who turns down the opportunity to be happy because of some other person's moral code is a fool. It's fine to say "oh you should leave/divorce first", "overlaps are tacky" etc, but you don't know what someone else's personal situation is, and again, you're applying your own moral values and judgements to someone else who quite likely doesn't share them.
Most pertinent example I can think of is a male friend who was having a two year affair while still married, but the marriage was totally broken, miserable, and irreparable because of his wife's behaviours. He instigated divorce proceedings, he's still with the "OW", and they're blissfully happy. I see no reason at all why he should have spent those two years needlessly being miserable by denying his feelings for OW just because there is a timescale attached to divorce.
The same thing happens on MN with posters being really judgemental about people who leave one long-term relationship, and are in another in very short order. There seems to be no recognition of the fact that people can be utterly miserable in relationships for years on end, finally have the fortitude to leave them, and that is such a relief that there is no need whatsoever for a "grieving" period because simply being out of the relationship is comparative bliss. They then meet someone new within a few weeks, and they are not "on the rebound" because they did all their grieving while still in the previous relationship. That's how it's invariably painted on MN though, especially when it's a man who leaves his wife and then has a new GF in short order. This exact thing happened to me. 20 year+ relationship, left, met my current partner within 6 weeks, been together nearly ten years and still happy. I couldn't care less how anyone else feels about that, because I'm perfectly comfortable with it.