How do you get married?

Just take them on missions and eventually you'll have the option to marry them.

That's not true Krunal. You can be a ruler and get married. I was Lu Bu in Land in Chaos, and I married a custom female character. Right now I'm playing a custom female character (A different one) and I had her marry a custom male character. Both Lu Bu and the female character have married and they are rulers. Emperors as well. So you can get married while being a ruler.

Furthermore, you don't need to do all that challenging and oathswearing with everyone and their grandmother, you just need to get a ton of points from missions. It took me getting most of the frickin' map, but by Megas my character is married.

SonGoku11, you've got it right. Keep going on missions with the person you want your character to marry and eventually it will happen. If you're in a real hurry, play as the character you want your character to marry. If you want Lu Bu to marry Yue Ying, play as Yue Ying for a few missions. If you want a custom character to marry Cao Cao, play as Cao Cao for a few missions and get as many points as you can. That's all there is to it.

Same goes for oaths. Once a character of the same sex gets enough mission points, then you can swear an oath with them.