I was thinking of the sex episode of Bill Nye Saves the World. He says:

We used to think [sex] was pretty straightforward. X and a Y chromosome for males, two X's for females. But we see more combinations than that in real life. And even for people with just two sex chromosomes, hormones can vary wildly. So can anatomy. What makes someone male or female isn't so clear-cut.

Clearly he and those who think like him think its more complicated than the absence or presence of the Y chromosome. Since they seem to have gotten hung up on chromosomal abnormalities, genital size, and hormone levels, I go to the ultimate question: can you get pregnant? Can you impregnant another member of your species?