Despite the messiness of biology it is, has always been, and always will be a reasonable statement to say that "boys have a penis and girls have a vagina". That

Despite the messiness of biology it is, has always been, and always will be a reasonable statement to say that "boys have a penis and girls have a vagina". That really shouldn't need saying. Of course, the meaning of the statement depends on the meaning you assign to "boy", "girl", as well as "penis" and "vagina". But in the semantic/communication environment in which we operate, the statement is reasonable because most people define those words such that the statement evaluates to usually-true. And usually-true is what we work with in biology and language; it's not computer science, it's messy, so approximate language is appropriate.