Heterosexual is an orientation which is, in males, based on male sexuality. This, at its most simple, is the desire to penetrate. Femininity is the set of gender signals used by heterosexual women (ie, those who wish to be penetrated and are attracted to masculinity) to advertise their female sexuality. Heterosexual men respond to these gender signals. At the same time, to advertise their own male sexuality, they express masculine gender characteristics.
Gender is clearly about more than just sexual attraction, since it has reproductive components like nurturing, protecting, desire for children and so on. But its biggest, most important and most obvious factor is the signal it sends out about the sexuality of the individual. Such signals and the gender binary can even be observed in populations where the people wear no clothes and do not follow Western patterns of monogamous marriage, eg the Zoe people of the Amazon.