He is being so dumb here but he always wanted to be a house husband. We know that she isn't actually trying to be with MC but needs him for some reason (probably pregnant and needs him to take ownership for her social status).
And the MC's whole character is a need to have a "family". In this case he would finally get his wish which is why he is running forward ignoring all the signs.
Knowing Sasuga Kei, the MC will end up taking care of the ex's kid together with one of the sharehouse girls, while the ex will fly the fuck away to Mexico without him.
Bro you have 3 chicks worried about you, they are far better than your ex, his ex is hiding something maybe she has already pregnant with random guy sot that's why she wants to get married quickly. Please don't be a loser
She's probably pregnant with the other guy's kid and is doing everything she can to speedrun a marriage with MC. Talk about falling for her shit without even a second thought.