When we (academics) do experiments on people however trivial we usually have to go through ethical clearance, get informed consent etc. I think its skipping that part that people are uncomfortable about.
You do realize that you yourself conduct such "experiments" on your friends every day? While making conversation in the lunch room you ask, "Hey, anyone wanna see Planet of the Apes tonight?" That elicits a lukewarm response, so you then ask "Well what about How to Train your Dragon?" You get a lot of interest in that one, so next time you ask about watching movies you're more likely to make suggestions where they can bring along their kids.
I think the dividing line between when you need to get informed consent is when the experiment begins to make people do things they wouldn't have done anyway. Tweaking how people get paired up for dates is fine if they were looking for a date anyway. Forcing them to go on a date when they weren't planning to would require informed consent (and probably compensation).