From Teletraan I: The Transformers Wiki

Despite being robotic lifeforms with generally non-sexual methods of Reproduction, the Transformer species has almost always been shown to include both male and female gender analogues, at least mentally if not physically. Female Transformers are comparatively rare, but are considered an official part of the Transformers franchise in every continuity.

(A Note on Sex: "Gender" is a mental and cultural rather than physical division and does not necessarily imply sexual reproduction. See the Reproduction and Female Transformers links above for more details.)