Pretty sure you're confusing Rosalind and Victoria. Rosalind definitely got fucked by Ian before you even meet her.
But with Victoria there are just so many strange coincidences when she and Ian just happen to show up together or he shows up at her house. She also has no problem just wearing a towel in front of him. Ian obviously has no problem fucking older women. Even Ian getting the MC the job could be part of it. Victoria not wanting to get into a new relationship now that the MC is out of the house. Ian not being to broken up about Mina, maybe because he is in another relationship with someone he actually loves? Etc. etc.
All of this is very much conjecture and TD most likely will be somewhere between laughing and shacking his head about this, but sometimes it can be fun to theorize.