NPC flirt.
The way I see it, it should depend on the personality. Some people are more flirty than the other. Say, Sera or Bull can make a move on you, while Blackwall and Cullen would be more cautious and would expect some hints from you.
Uproar about Anders was mostly because how it was handled (no polite letdown, hissy fit and rivalry points). That's because Anders is a douchebag though, and people did not expect such behavior after Awakening. The timing was awful too. "Hey, I just killed my boyfriend/close friend, how about sexy times?!" Creepy. Isabela hits on you too, nobody was offended, it's Isabela, she flirts with everyone.
Ninjamance was bad, though. (No, Kaidan, I didn't give you mixed signals, I was just polite. On later playthrough I had to jump through several loops to avoid this situation completely.)