If one knows in advance before they chose to marry someone it makes no sense to expect them to end their friendship unless you suddenly learn after the fact they were more than "platonic friends".
In that case they would have misrepresented themselves/lied to you regarding their past. That would raise trust issues.
I'm not a big fan of being best friends with exes. It's my own personal preference and I would most likely not date or marry someone who felt the opposite. Nevertheless there is no "right" or "wrong" involved here there is only "agree" and "disagree". Everyone is entitled to have their own "deal breaker" list and preferences.
People who (naturally agree) on things they consider important tend to get along better than those who don't. Beware of "red flags"!
The goal is to find someone who shares your same values. Like attracts like in the long run and opposites attracts divorce attorneys.