Women who demand to be allowed to finish get called unpleasant, gendered names. A man can be "powerful" or "confident", but a woman behaving the same way will be called "bossy" or worse.

And every single time she gets interrupted, she is forced to make a choice: is this the time I'm going to insist and risk getting a reputation for being unpleasant, or am I just going to let it slide because they don't mean it unkindly.

Try this: the next time you see a woman being interrupted, you say, "Hang on, I wanted to hear the rest of that." That not only sends the message that interrupting isn't ok, but it also saves her the effort of making that decision yet again. It's no surprise that they often end up choosing not to, solely as a matter of picking their battles. Ending that unconscious sexism shouldn't just be on them.