different requirements based on gender

Is it Illegal for An Airline to Fire A Female Flight Attendant For Gaining Weight But Not A Male Flight Attendant?

I am a female flight attendant. I gained 7 pounds over the winter and was asked not to come back to work until I had lost all 7 pounds. My employer said it negatively impacted my interaction with my customers. My male colleague that flies the same route as me gained 10 pounds and can barely fit down the aisle and is still working. Have I been the victim of sexual discrimination?