Maybe your dad was a fan. Maybe you just grew up in Utah but were never LDS. Maybe you went to BYU or used to be LDS but later changed your affiliation or activity level. Maybe you're an active Mormon who does the church in an unorthodox way. Maybe you're a non-LDS dude not from Utah who was just drawn to the passing attack under LaVell. Maybe you liked Jim McMahon or LaVell or Bronco or Taysom and started following the program.
Frankly, it's kind of bizarre that you can't understand why a person could simultaneously be a drinker and a BYU fan.
Cheering for BYU football is not the same thing as cheering for Mormonism.