Frank seduced Pauline, who was dating Hank, and convinced her to dump him. (Yes, Frank is a d*ck) Things happened, and Pauline somehow got pregnant ; Frank proposed to her, and they eventually got married.
The thing is, Pauline's child was already born, and Frank wasn't there because you know, being a conductor at the Opera and working 5 hours a day is hard. Hence he discovered the child's name only when Pauline came to live in his house ; he thinks Hank might have chosen the name (Pauline and him were still living together), because it is awful. (The awful name will not be revealed here, to avoid hurting anyone bearing it.)
Now Pauline and Frank are living together. Without their child, who stayed with Hank.