I would say that humans are sexually dimorphic in the same way that humans are bipeds. if you attempted to make a serious argument that limb agenesis implies that we’re a variable-limbed species it would be obfuscating rather than illuminating.
99.8% of all matter by count is either hydrogen or helium, are atoms dimorphic?
No, that is not a good analogy at all. It's so poor an analogy that it's challenging to interpret this comment generously. I think you might be arguing facetiously to make a different rhetoric point than the literal content of your post, bot I will respond to your text literally.
Humans have a wide variety of biological variation in metrics we think of as linked to "biological sex" and those metrics are accessibly mutable. Even within the Olympics, the natural variation of these metrics within cis women is a famous topic of debate (Imane Khelif, Caster Semenya, etc.)
Bipedalism is something which varies very rarely and is especially not accessibly mutable.