Being nice and being douchey aren't mutually exclusive concepts. You can be nice to your SO and your future family while being douchey to everyone else. In practical terms, that means you always prioritize your SO and your future family over the needs of strangers.

In other words, be a Tywin, not a Jon. The world can be rough and no one wants their closest ally to stab them in the heart over moral quandaries.

You don't have to actively go out of your way to be douchey or condone douchey behavior by your SO, but there are way too many dudes out there who will bend over backwards to please their boss, friends and random strangers and then be too tired to take out the trash and look nice for their SO when they come home.