Nah, It was already mentioned during the conversation in a resturant between Him, Ayase, Yuka, & Her. After he find out his sister were in a bad state and want to thank her for protection Yuka. So, he invited her and that's how they talk about what happened.
(It's just you, who missed that part).
Also, game is not ruined it's up to decide who you want to love or not. I know she is a sex worker but just look at her situation or how she is living. If you can't love her then atleast be a friend who can support her.
Even, I was not happy and who would be? Life is not same for everyone. That's the reality.