Why do women lead men on when they have a Boyfriend and flirt with them?

I recently went through this yesterday. She always constantly giving my signs of flirtations and batted her eyelashes. Later that day, I found out she has a Boyfriend. So I'm like okay, whatever but it really sticks in my craw when they do this to men. Honestly, I don't think it's right. Mind you, this is only my own opinion of what I think. I definitely have had this happen before but it was at least 2 years ago. Maybe I should at least be thankful that I'm single, so that I shouldn't worry about it (which I'm no longer going to do).