Why does Japan not take relationships / marriages seriously?

False premise. Japanese do take relationships seriously. After work you'll find millions of them going together to the bars and watering holes to promote business relationships - spending hundreds of billions of yen every year. There is some truth that in offices they waste a lot time to _look productive_ and not go home before their boss, because it would look like they are slacking off. But Japan is the #3 economy in the world - and they wouldn't be that way if they were really what you portray.

As for family, raising the kids is largely seen an a mother's task.

Could they do more to promote better relations between spouses? In some cases yes, but you could say that about nearly any country. They don't have half their marriages end in divorce though as in other countries - probably yours.

Because love and marriage aren't the only things in life.

Divorce is very rare in Japan. If you're talking about the men's propensity for feeling up women on the subways, and taking trips to Thailand for the prostitutes, that is tolerated, and doesn't usually break up marriages. The Japanes do NOT "goof around". In fact, most didn't take the days of vacation they had coming to them, so the government had to pass a law requiring them to take vacation. 

If that's the impression you have of how things related to that stuff are in Japan, maybe there's some truth to it, though I think generalization could often be misleading and that's not really true from what I actually see here in Japan. 

I'm not sure that's actually true.