How to legally host fights in your house? Why is it legal for us to "practice" at my boxing gym but illegal for us to "practice" at my house?

The thing is that one man's "practice match" is another man's "assault and battery." You need to think about this on two levels.  First, boxing matches, martial arts tournaments, ultimate fight club matches, and the like are conducted by legally sanctioned organizations that comply with the laws and regulations and insurance requirements of specific jurisdictions and governing bodies. Have you ever wondered why so many boxing matches are conducted in Las Vegas? That's because the State of Nevada and the City of Las Vegas have specific laws that allow properly sanctioned matches to be held. Every jurisdiction has its own laws and regulations. Just because it can be done in Las Vegas doesn't mean it can be done in Texas. Secondly, you need to think about the huge liability you would face as a promoter not if but when someone gets seriously hurt. In many jurisdictions it doesn't even matter whether participants sign liability waivers and releases. Certain rights just can't be signed away, particularly when it's a wrongful death claim and it's the relatives who are coming after you. Hosting a fight club on your own property is just a bad idea.