Male and female both have specific duties that they are obliged to during certain circumstances in Islam. This has nothing to do with inequality and has everything to do with the plain differentiation between the sexes.
For example, there is no physical reason (excluding illness, impure state, etc) for a male to stop praying. While for the female, she is obliged to take time off on a monthly basis during menstruation. The same goes for fasting during Ramadan.
Males cannot cry foul because females are given "time off" on a regular basis. Similarly, females cannot cry foul for other obligations that are imposed upon them and not on the other sex.
Sexism arises when a matter of negative inequality is there. In Islam, the superiority or inferiority of a person versus another is based on their spirit (obligation fulfilled, good/bad deeds, etc) Males and females are thus equal in fulfilling their obligations.
Going back to the example I gave, a female during menstruation unable to pray is as equal to a male who is praying. Both fulfill their obligations.
Muslim males do not have divine obligations imposed upon them similar to females whose physiological being is different. A male does not menstruate, get pregnant, etc. Males and females are different - and they are beholden to that.