Can one person be responsible for making a good decision for another reasonable, normal adult?

A couple friends and I have had an ongoing dialogue about the following hypothetical situation:

Two people, A and B, enter into a physical relationship. A expresses romantic feelings for B. B does not return these feelings and explains as much. Should B refuse to continue the physical relationship out of regard for A’s feelings, or allow A to make that decision?

I think if the physical relationship continues, B is clearly taking advantage of A, who is clearly making a bad decision. If A proves incapable of making a good decision, I think B becomes culpable for A’s feelings and is obligated to make the good decision for both of them.

A friend of mine argued that this is patronizing and A should be able to make whatever decision A wants.

What do you think? Also, does your answer change depending on the gender of A and B?