I really don't see how else one could interpret a woman explaining how she was sterilized and then immediately saying she's a monster too.
Or maybe it's the killing?
- Bruce Banner: You sure? Even if I didn't just...there's no future with me. I can't ever...I can't have this, kids, do the math, I physically can't.
- Natasha Romanoff: Neither can I. In the Red Room, where I was trained, where I was raised, um, they have a graduation ceremony. They sterilize you. It's efficient. One less thing to worry about. The one thing that might matter more than a mission. It makes everything easier. Even killing. [she hesitates a moment] You still think you're the only monster on the team?
The movie really does treat her like a sex object. Captain America tells hulk that she's a real good flirt up close and bruce banner is like "wooah what do u mean up close." Every conversation about her is about whos been with her or who will get with her.
When in doubt, pivot.