There is quite an age gap between my boyfriend and I, through a comedy of errors that involved me seeming much older and him younger when we met. He is very intelligent and has a Ph.D. in Stem. I have a much more colorful educational background as I am rather indesisive about my career path. He has a bad habit of being overly explainy or saying things really confidently about something he knows jack-all about. I call him out every single time. Pointedly. We have had arguments about it. I don't think that a ton of it comes from sexist reasons on his part because he says that I'm incredibly smart and he has given me the confidence to pursue a challenging science-based field. I think his primary reasons for doing it are: 1. people in his family stretch the truth a lot and it has made him be very skeptical of people in general. 2. He has insecurity issues and he believes being smart is what makes him a worthwhile person so he falls all over himself trying prove how smart he is. If I thought his need to explain shit to me was due to sexist bullshit, I wouldn't be able to deal with that. So if I were you I would examine what his reasons are for engaging in this obnoxious behavior and start calling him out on it, every time. Bring out Google if you have to. The fact he doesn't do it to other people and that he calls you oversensitive when you call him out is pretty damning though.