There's no binary in aptitudes or abilities but that doesn't mean that there's none in physiology. Men have a part of their brain that controls their penis. Clearly, women do not.

Overwhelmingly, Viagra doesn't work on women because female sexual response tends to have some major differences from that which is typical for men. Although it's still too simplistic to say "Men are like this and women are like that" there are some meaningful trends. We don't know for sure what part of it is innate and what part is socialized in, but in this culture, women tend to have responsive desire more often than they have spontaneous desire where men's tends to be more spontaneous. This doesn't mean women are "less sexual" than men - it means that their sexuality tends to be slightly different, for many women, a good bit of the time - something that I think is useful and important to note.