TBH I think that’s Kurt’s plan & why he is following along with this wedding. She trapped him with the pregnancy and forced him to propose. He could of escaped at any moment until now, but I think he has realised that marrying Sarah gives him full benefits to all her assets and everything will be 50/50 if he decides to split. Plus he genuinely loves Fox and I’m sure he would never abandoned him. We often take Kurt for a fool, however I think Sarah intimidates him and lowers his self-esteem, but once things become 50/50 he’ll regain some power (& possibly some hair)
and he’ll feel better equipped to take control of this situation (hopefully!)