He made bold declarations of taking responsibility recently, but when she brings it up here he's like a deer in headlights
Because she had already denied the offer before, and MC didn't expect her to suddenly bring that talk again.
MC is ready to take responsibility for all the girls he had impregnated specifically for his Level-Up plan orchestrated by his Onee-san, but only those who want his help. Majority of the girls (who are prostitutes), didn't either ask or deny help from MC yet.
There is also the point of MC at that time fell in love with Ratri, but she dumped him. And after that he reconnected with Marie and is treating her as primary.
Keep in mind, before his Onee-san started the whole plan to Level-Up MC, he had only impregnated Ratri, Marie, Unknown & Onee-san, aka., just four people.
Ratri had already dumped him, Marie is in emotional conflict because of her lifespan issue
(though she is also the only who currently doesn't know anything regarding MC), Onee-san doesn't want MC's help instead she wants to help him Level-Up more, and the 4th one is currently unknown to MC.