I think the question, "Is insulting a woman de facto sexism?" is not without merit. It does seem many people think so, just like any criticism of Israel, is deemed by some to be antisemitic. I mean, was Trump's "low energy" about Bush or "little Marco" about Rubio sexist? They were both obvious personal insults/attacks but sexist? I believe folks might be too quick to misinterpret personal insults as automatically sexist.
A better question would be do we want a POTUS that does this to both men and women?
I honestly think Trump just doesn't respect anyone who isn't Donald Trump. He finds something for everyone, even guys like Mark Cuban.
That said, there is a particular flair to how he disrespects different people. His shit is generally lazy, and he goes for the easiest targets. Categorically sexist things are among them.