Katsuki and Izuku have talked about this before. They both had their flaws, but Katsuki didn't think it would get this bad for Izuku.

Izuku had a habit to smoke. But now that he was pregnant with a pup like Katsuki always wanted, Izuku didn't stop smoking. Smoking was bad for their pup.

Katsuki had to put a stop to Izuku's bitchy behaviour. He was acting more bratty like these past few months. He knew Izuku also didn't really favor pups, but had agreed to bear one since they were already in their 20s, the time their alphan and omegan nature was desiring pups.