I'm not going to buy my fiance a new car if I total his.
That doesn't seem fair. Why wouldn't you replace his car if you total it?
I don't think it's my responsibility.
It's wrong to not replace your fiance's car if you total his. You should take responsibility for your actions, and sometimes that includes having to pay for or replace something that you damaged.
We are both covered under our parents' insurance so I'm not paying for shit.
Relying on your parents to make up for your mistakes is wrong.  They will be the ones forced to pay for increased premiums thanks to your mistakes. Don't you feel like thats unfair to them?