woooo. umm, sorry to burst everyone's debate if anyone comes upon my comment but the novel was a lot more- it was less than this. this is one way to interpret the old man, but in the novel a clear reason was omitted. he was made to be a scumbag to be hated on. when Maomao revealed that you can drop a courtesans price with such methods, her father was the realization of those desire filled people. he is hated on by Maomao. how her father slept with her mother was kept vague, but I could see this happening.
the novel was written using Maomao's perspective so it's safe to assume that it was biased. in the novel it seemed the reason why he acknowledge her as his daughter because she was just as interesting as her mother. he's lived his entire life seeing people as game pieces, wanting to keep the one person interesting by his side just for the sake of it
even with the bias- it's not hard to believe that he was a desperate father figure separated by circumstances. but this is some whitewashing to say nonetheless (I'm using the non-racial definition)