I think that when she refers to herself as a monster, she is referring to what the KGB has turned her into, not that she is infertile.
Sort of.
The sterilization was part of a “graduation ceremony” from her assassin school. So it was voluntary.
She was expressing sympathy to Banner (who is pretty emotionally f’d up, as we all know) because she too had voluntarily subjected herself to a medical procedure that altered the nature of her being and turned her into something destructive. I actually thought it was an effective and well done scene. Especially since it’s the emotional bond she develops with Banner that helps keep the Hulk under control.
And, in seeing the trailers my original thought was that she was just using honeypot tradecraft - deliberately and cynically manipulating Banner so the Hulk could be kept under control. Altho that sort of thing would have been consistant with the Black Widow character from the comics ( and from the other movies, remember how she interrogated both that Russian general AND Loki in The Avengers) I was glad that there WAS a real emotional bond between them. It’s going to make the Phase III movies they’re in better, IMHO.