I know what you mean about that weird vibe. Obviously marriage isn't an option for my relationship, but I know plenty of married couples who are into BDSM on various levels. It's like she's buying into the most restrictive "monogamy means pure, chaste, boring sex" hype out there (which is unfortunately prevalent in poly communities; the number of "we're more ~evolved~ than monogamous people" bullshit artists I've had to smack down in my time is truly staggering). Poly and kink overlap a fair bit, but there's no reason monogamously married people can't be kinky too.