Is genitalia the extent of biology? Like I said, boiling biology down to genitalia is overly reductive, as it ignores things such as genetic variations and hormone levels, which also fall under the banner of the biology of sex.
I'm sure we can investigate genetic variations and hormone levels but these require blood tests: you can't assess these things just by looking as far as I know.
Simply looking at what's situated between the legs is far more convenient and pivotal in distinguishing between male or female. I'm sure we can look at other things but our genitals will never not be in the conversation.