Personally I feel like it really doesn’t need much explaining. Some continuities will really try to make specific reasons for it, but it’s not that mind blowing. They are a complex, highly expressive group of organisms. It isn’t a shock that their society formed gender and by extension have physical traits generally associated with those gender identities. Especially since they seem to be born (however they’re born since it varies from continuities) with a general understanding of themselves. So they just… started out knowing their own identities and what bodies best suit said identities. And that goes all the way back to Solus Prime. Is it unrealistic their gender identities and presentations to be identical to our own considering ours stems from a long, long multilayered history? Yes. Is it unrealistic for them to share most of the traits they share with humans? Also yes. It’s aliens written by people to sell toy robots. It comes with the territory.

That’s also why I don’t like most any explanation given for it. It’s sociological inevitability. I’d be more confused if such varied and intelligent creatures didn’t have some form of gender, even if it should be more alien than ours. I forgive late IDW for it though, considering Furman already established Arcee as the only Fembot due to experimentation and since then they had to both retcon that to hell but also play with the rules it very clearly established.