So bummed

So my fiancé and I have a pretty good relationship and it's very rare we fight. I consider myself truly blessed! I could go on and on but anyways we got in a fight last night and we hardly ever do like seriously maybe twice a year and last night fight was pretty bad like not physically but mean things were said now today he's saying he needs a brake and he just isn't invested in our relationship like he used to be and so on. He says maybe he should leave. I just don't get it all over something stupid! What did I do wrong? I didn't think I had done anything wrong and thought it was just him being him till tonight when we tried to talk and he said our sexual life is slacking which it is and he just isn't attracted to me the way he used to be. Not because I'm not pretty he still finds me pretty but cause I'm not super smart like him I guess. He wouldn't give me an answer. See he's super smart and learns way above average. Growing up he had over a 4.0 grade average and I've always been lower. I don't get why it's bothering him now? I have been the same person for 5yrs. I just don't think it's worth throwing away 5yrs,a engagement and we have a kid together away cause our sex life is slacking or I'm not super smart! He says I don't understand his job! Well let me tell ya how job involves a lot of math and number and computer work and so on. He says I don't put in the effort. Why is he wanting to give up?