By 'suddenly thrown' you mean the promise she made in captivity, in the shower 3 episodes ago, right? Yeah, amazingly sudden... nobody saw that coming...
Wow, you two have
no memory capacity at all. Jensen's very first sex scene in S1E1 was with
one of Cynthia's lovers - the one she didn't choose that night. Jensen was taking
her leftovers.
Right from the get-go, you met Cynthia going clubbing with 2 of her girlfriends, and meeting up with her older lover who is also her mother's close friend. Cynthia was always, from the first moment you met her, a rich kid socialite who sleeps around with a whole bunch of women. She flirts heavily with Mei at first sight, when Mei and Nicky met her at the clothes boutique.
If you somehow missed all of that, then honestly, words fail me.
If you can somehow miss something that obvious, then you certainly have no chance at all in spotting the subtext of how the wealthy girl with the very busy and successful Father and his second wife, feels they just don't seem to have time to give her the attention she so craves, and so she has developed the habit of taking all the love-substitute she can get.
That scene in particular was an important part of opening up some specific path options later, specifically for Mei, but please, don't stick around for them. Leave that to people who don't make idiotic snap judgements based on the first impression without any knowledge or care for why it is there, and the disrespect for the writers to express it.