Legal aspects of marriage before/after?

My boyfriend refuses to get married because he says he doesn't understand what will happen legally. He says he owes the IRS money, and wouldn't want to burden someone with that. I have been searching allover the web for what becomes jointed when a marriage legally takes place, but some stuff says "this happens" and other sites say "you can choose to do this". Now it has me confused. Can someone put it simply in plain English what happens financially, and legally? Don't redirect me to a website, lol.