We should run a little experiment.
Step 1: Go find a person that you don't know very well, but know for a fact is in a relationship with someone else.
Step 2: Grab that persons ass, and I mean really play the fucking bongos on those cheeks.
Step 3: Report your findings to the classroom.
My hypothesis: you'll wish the person you did that too had as mild a reaction as Cindy.
So no, the expectation isn't that everything you do should lead to sex, because actions have consequences. And unsurprisingly, or perhaps surprising for some, molesting casual acquaintances tends to be a turn off for like 99.98% of the population. It's perfectly OK, encouraged even, for their to be negative reactions to Ian or Lena's less socially acceptable behaviors.
Spoiler Alert for if you decide to pursue Cindy: