To be honest in my first playthrough I was also a bit surprised (more shocked) about the turn in the Lena / Seymour relationship. I’m also not 100% sure if in that version of the game I had the possibility to rejected his offer in the restaurant scene (but maybe I’m remembering it wrongly), for sure I couldn’t reject the vibrator.
To be fair, my first playthrough was a bit fast and I didn’t put so much attention into every detail of the story and think back to the first meeting of Lena and Seymour in the art gallery it’s pretty obvious that Seymour has some dark intentions, so I rejected his offer in my other playthrough.
On the other side the first dinner in the restaurant and the two photo shooting scenes are more or less professional (if you do the second one without Axel). It’s clear that Seymour is a powerful man, how is always in control of the situation and always has the overhand. So, you can see I’m still a bit shocked about the events in chapter 9

. Maybe I have to replay the scenes and have bit a better look into the details, if I have missed something that make Seymour's reaction so obvious.