Heartbreak with friends with benefits- read description before voting

So I'm watching a new show, and it got me to thinking.

Say someone (Abby to give them a name) is friends with benefits with someone else (Sam). From the beginning, both agree that this is PURELY a sexual thing. Sam starts specifically saying that he is starting to fall for Abby, but Abby continues to state that she is not interested in a romantic relationship. They have both been open about their respective feelings. Eventually, Sam gets his heart broken. Who's fault is it? Is it Abby's fault, for continuing to have sex with Sam, even though she knows Sam is in love with her? Is it Sam's fault for continuing even though he knows Abbey doesn't want what he wants? Is it both? Is it neither?

Bonus question for the comments: would your answer change if the genders were different (male wants just sex, female wants romance, to give one possible example)