I think we assume things because our framework for relating to one another is so solidly instilled in us from infancy onward that we tend to think it is necessary to know. And sex is the absolutely primary element of that.
Our language requires that we choose between two gender-specific pronouns in referring singly to one another. Many people are insulted if we misread their gender (even when they don’t help us get it right), and our mistake makes us appear either rude or ridiculous or both. Those are socially uncomfortable feelings, and so we try to avoid them.
I think many of us also subtly alter our posture with respect to one another depending on perceived sex and/or sexual orientation. Again, those things are built into us from birth because of how we are treated and taught.
If we don’t know whether someone is male or female, we fill in with assumptions based on the clues we have.
Of course, most of the usual clues are missing in an online environment, so we make inferences based on allusions and analogies to real-world traits. But they can be either unintentionally or deliberately misleading. Here just as in the real world, many people don’t have a sense of how they come across to others, and some cultivate the ambiguity. Culturally and socially, we are really only beginning to learn how to interact as virtual entities in a created environment; in another generation or two, when there’s no one left who didn’t grow up with computers, there’ll be new conventions for many things that are awkward now.