I think you misread Kyu's post. He doesn't have anything against mixed marriages (as far as I know
) but merely stated how Japanese society as a whole thinks about it.
So yes, it IS a taboo! Even more so that children from mixed marriages have more trouble fitting in, something my girlfriend and I have discussed at length. Especially marriage with a foreign man are taboo. Just to demonstrate this point:
My g/f and I have been looking into a possible marriage and the kind of paperwork that would need to be taken care of. Lo and behold! Lots more red tape and paper pushing when in a mixed couple the foreigner is the husband. We suspect they do this solely to discourage mixed marriages. So in short, it's much easier for a Japanese man to marry, oh let's say a Filipina than for a Japanese woman to marry a European man.
Another example from when we started dating: she was giving me a ride with her car and we happened to cross one of her coworkers who was on foot. I had to lower my seat immediately to make sure he didn't see me and would start gossiping about her affair with a foreigner. Worse, once I went to her office on business and she made me pretend not to know her for fear of the reaction of her boss. She did tell a few female coworkers whom she trusts, but in general it's been kept hush-hush for over a year. Now, fortunately, she has come out of the closet for the most part, except still not towards her boss.
So yeah, pretty bold move by TOHO here!