Without diving too deep into the background of the Joss Whedon*/Avengers brouhaha, let me give my very quick take on the story aspects that are causing so much disruption in the fandom.
Joss Whedon didn’t write the backstory for Black Widow, so you can’t blame him for it.
Logically, based on the time frame and circumstances of Black Widow’s development, it makes perfect sense for this to have happened. Her entire childhood was spent teaching her to kill, then, when she reached adulthood, when she might have though of becoming more than a killer, the ability to create life was taken from her, leaving only the power to take it.
Finally, having moved beyond her upbringing and training, she is confronted by the possibility of an existence of a life that can something more than killing. Have you noticed that many times she doesn’t kill, merely incapacitate?
Realizing that there is something more out there for her, she begins forming tenuous bonds for purposes other than espionage or assassination. Captain America, Hawkeye, and finally, yes, Bruce Banner. Not the Hulk: Bruce Banner. In Bruce, she sees something not so different from herself: a person whose capacity for close connection to others has been taken from them. Is it so wrong to think she would be attracted to him for that, in addition to an otherwise normal, emotional attraction?
As for the use of the word “monster” to describe herself? It is a perfect description in the context of the conversation it was used in. Bruce Banner can wreak great damage on the world and on those who get close to him. Is Natasha so different? She spent her life committing untold death and destruction and chaos on the world, unable to become close to anyone because she believed–she had been made to believe since childhood–she had nothing to offer anyone other than her skills as a spy/assassin.
In fact, the idea that she was able to connect with Bruce so deeply as to admit her fears and needs and wants–not for children, but for an honest connection–speaks volumes for the mental and emotional strength she has within her. She is far more than just a killing machine. Though she cannot bear children, she can still help inform life.
And her acknowledging it proves it.
But that’s the way I see it from a story point of view.
*We’re not going to discuss what some people think about his evolution–or lack thereof–as a writer.