Here’s the article on Wikipedia: https://en.wikipedia.org/wiki/Motte-and-bailey_fallacy

In short, this fallacy is when you combine an argument that is easy to proove (motte) with another that is more difficult to prove (bailey). When someone refutes the bailey, one would say that they are refuting the motte (which isn’t true)

The first example given is what I’m talking about in reference to gender:

An example given by Shackel is the statement “morality is socially constructed”. In this example, the motte is that our beliefs about right and wrong are socially constructed, while the bailey is that there is no such thing as right and wrong.