[fork1]

There is plenty of brackets so all can play at their respectives levels, separation in different leagues for collective sports, different levels in individual sports, etc. You can have fun in your local amateur <whatever>, but there would not be any point in a game against professionals. And at the top level, olympics or international competitions, there would be no match in mixed genders for most sports.

[darkerside]

There are weight classes in weightlifting, boxing etc

[inkblotuniverse]

Men would win every weight class due to stronger ski, connective tissues, different and more efficient bone structure...