Condescending Bias is such a good word for it! I know exactly what you mean with that tone.
What always drives me batty is that, as a history lover, I feel that I have a decent perspective on the actualities of love and marriage rather than the rosy portrait so-called "traditionalists" like to paint. Women were abused and subjugated universally and across the different marriage systems -- and besides, up until (and including!) the modern century our so-called monogamy was anything but. Men broke the bonds of fidelity with mistresses and paramours... that was just the expected norm and the women weren't to complain about it or expect anything different. Wives were with whom you formed a family and household, lovers were with whom you enjoyed yourself. This was the status quo in all "monogamous" societies! Marriage was not about love it was about domestic economy and politics.
So what could be more fair than a marriage in which expectations are negotiating and all parties communicate honestly their desires and wishes?
Anyway I will get off my soapbox now
I just get so worked up with these sort of false traditionalist arguments being used against gay or poly marriage!