Not entirely true.  Try reading it differently.  She had to have been open to it to consider the possibility, and some people need nudging.  She even goes on to say that she was turned on by the whole thing, and that she was super into it.  Then, this outcome totally hit her by surprise:  for all she knew, things were peachy keen and she enjoyed herself.  But as for whether or not she was actually OK and wanted to (perhaps expressing she was game for it before but confiding that her insecurity makes it difficult) is entirely OP's call, not yours or mine.  Given the way the story reads, I'd wager that she was already somewhat open to it to begin with.  The true failure here is their collective failure:  rushing into it without much of a plan of attack.    

We can fill in what his convincing looked like with our mind's eyes, and I think a lot of the ill-will towards OP's boyfriend has people imagining him begging daily until she just rolls over and says "okay fine."  This is just a couple of rookies who messed it up, which is NOT uncommon when bringing in a third person.