I don't think it should be only the woman's responsibility. I'm sure there are cases where a woman lies, says she's using birth control and isn't, but I really doubt that makes up any kind of significant percentage.
I agree that unplanned pregnancy and its offshoots can be really unfortunate, but disagree that its all the woman's fault. Not all the man's fault, either.
Dan Savage had a great response in a recent column about the man's role in family planning: