Scenario #1: You are at the bar, you find a woman, bring her home, do the good ol' haystack dance and then it turns out she wants more.

Scenario #2: You are semi-friends with a woman online, you meet, you bring her home and do the good ol' haystack dance (I love that expression) and then it turns out she wants more.

Etc. You get my point.
My question is: WHAT TO DO?
Of course it should in theory be as easy as saying "Well woman, I only wanted some sex, and I thought you did too so whatever", but aren't there better ways? What are your experiences with this problem, and how do you usually deal with it?