@yitzyshalom-
There’s a HUGE difference between typical Halachic Shailos and this case. Typically you’re right, if you hold of a certain hechsher then that item is perfectly Kosher for you and might be not Kosher for me, that’s fine. My family got our first microwave because a friend “treifed-up” theirs with cholov stam, so gave it to us. To them they couldn’t use it, but we could- no problems there. Same thing with anything else.
The problem here is that the results don’t affect only the individual. Yes, according to one shitah the person’s a mamzer and according to another (s)he’s not, but what happens if- as happens often- that child decides to become a follower of the more Yeshivish derech? Suddenly now they’re considered a mamzer? The simple “I used to not keep Cholov Yisroel and now I do” just doesn’t apply here, and I would never enter into a marriage if one day my kids might have to consider themselves mamzeirim, or if potential Shidduchim might consider them mamzeirim.
@simcha613- There are ways to do it- including, perhaps, a pre-nup- but it would have to be written up differently so that there’s no ch’shash get meusah.