One example probably shouldn't set the stage for all. Any relationship takes an incredible amount of work, maintenance, communication and willingness to hang in there to do all that.
Having something in common from the beginning isn't a bad thing, even while it does have the potential to make things tougher, it can also have the potential to provide a good base. It's up to the couple.
I might see where Martindale hooking up with a gal who had only had PFAL could cause massive strife, I get that part of your points.
But like any relationship, twi related or not, both have to be 100% aware of the others' "issues" (I'm sick of that word) and deal. There is also the added good of some hearty discussions at the supper table sharing memories of all that stuff. "Remember the time Wierwille said _____ "and some great and necessary laughter to follow or perhaps some healing from being able to have someone that gets it. That's pretty valuable and easier in perhaps another dynamic where the other person just can't relate.
I don't think it's as simple as putting on ones dance card "if you were ever in a cult, I won't do the two step with ya".
Love doesn't come around very often and why not take it if it does, if both are ready for love and willing to do what it takes? And we can't judge a couple from one experience, as well, I don't think.
Hopefully if a couple are rockin it like they're "supposed" to, it's not about twi or any other such prior involvements, but rather about the two of them figuring it out privately and personally and if they do choose to offer something more public, it's done with an amount of tact and respect to each other, before the community with which they share.
Communication.