Sadly, there are far too many gymsyou could end up in where there'd be no difference in whether or not you were taken seriously.
There are many gyms that operate as health-club boxing type gyms for the bulk of the time, to pay the bills. So they would take you seriously- way to seriously for what is an aerobics class.
To be taken seriously in a "fight" gym you have to be serious; not acting like some movie clown but working during your work out and not just doing the routine. Work hard physically and mentally and you'll be ahead of the game.