Apparently I erred too much on the side of short and punchy. The scenario you outline seems like a special case of the point I was trying to make: namely, that changing minds in any way changes the basis of a relationship, and sometimes the relationship in its changed state will end up unstable. Even aside from your (entirely reasonable) scenario, there's unfortunately no shortage of people who seem to respond poorly to rationality as implemented by LW in its current state (as opposed to idealized always-wins rationality), which presumably shares a number of characteristics with the flavor the New York guys are teaching.

Heck, a competent instrumental rationalist in an unhappy relationship is if nothing else probably more likely to hack off the bloody stump of the partnership than a non-rationalist (being undeterred by sunk costs and unwarranted feelings of obligation), and I'd expect that alone to generate a correlation between new membership in rationality-oriented communities and the termination of existing relationships.

And yeah, the cult accusations are annoying, which was the other point I apparently failed to convey. I've seen firsthand what real cult psychology looks like. It's not at all pretty, and it's not what we've got going on here.