Mammals raise their offspring inside their bodies. That’s still a valid rule even though there are two exceptions, the platypus and the equidna, which are mammals that lay eggs. So, what you say is not true. Science has many rules that are still valid in the face of exceptions.

While I disagree with TERFs and agree with the spirit of your article, I find your arguments specious and unscientific. You are using the “marginal case” fallacy. In complex systems like living beings, there are always individuals that stray from the norm. This does not invalidate the rule. Otherwise, the induction method wouldn’t work and we will have to throw the whole of science out of the window.

Here are other examples of valid scientific rules with exceptions:

• Proteins are made of 20 amino acids — however, sometimes proteins incorporate other amino acids.
• Neurotransmitters carry information from the presynaptic terminal to the postsynapse — however, endocannabinoids act as “retrograde transmitters” and carry information in the other direction.
• Action potentials take place in axons — however, there are action potentials in dendrites, but they are very different from those in the axons.
• Information is transcribed from DNA to RNA — however, some viruses have an enzyme called “reverse transcriptase” that synthesizes DNA using RNA as a template. One of such viruses is HIV, which produces AIDS.

In all these cases, the exception does not invalidate the rule. Instead, the fact that they break the rule is what make them particularly interesting.