And you don't think there are fights over heterosexual couples or even bachelors under similar circumstances? Anytime there are sufficient money or possessions to be had people will fight over them.
In most states, however, if you leave everything to your spouse the only people who are going to be able to fight you over it is your children. If you're gay, in some states you could have brothers and sisters and parents involved as well.
You should be able to dispose of your possessions to whomever you damn well please, including your cat.
BTW, if any of you leave your assets to your cat, I love cats and would be happy to be your trustee.