I didn't say boundaries were a bad idea. Kinks should, in fact, be all about boundaries and trust, and for most people, they are. The OP ex doesn't understand that, and that has nothing do with his submissive kink and everything to do with him being a twat.
It's simply that you aren't the one who gets to set those boundaries for other people outside of your own relationships, because they are none of your business, and neither do you get to arbitrate on what is good or bad for two adults to do, with full consent, to one another in private.
It's not a case of 'kink = bad, abusive, harmful' and 'vanilla = good, healthy, loving'. The danger comes from coercion, manipulation and deceit, all of which is every bit as likely (and often a lot more insidious) in a mainstream/conventional sex life as it is in a kinky one.