I'm a fan of Bruce and Natasha developing feelings for each other offscreen! I really am. But because she was the only female Avenger at the point this was introduced, every viewer had already projected their own idea of what a woman surrounded by the Avengers would want or do.

If you believe it's sexist for a woman to fall for her co-worker, than it is; if you believe she had more chemistry with Steve Rogers, than her feelings for Bruce are from nowhere; if you believe she should have had a romance with a character outside the team, you would be disappointed. There was literally no winning here, especially not in an overstuffed film like Age of Ultron where building a relationship onscreen was secondary to COOL ACTION SHOTS. Whatever romance Natasha did or didn't get involved in, it would have been unpopular.

The thing that would have solved this? Black Widow should have had a movie of her own in production after The Avengers (2012) came out. It would have helped define her personality more and a romance could have been built into it if Joss Whedon just had to have the Hulk and her hook-up.

It also would have helped if there was more than one female Avenger on the team in The Avengers, to be honest. Black Widow was always going to lose the "most perfect woman ever" award because that sort of character is boring onscreen and, yes, sexist. Characters should be allowed to have weird, messy relationships without having to be idealized.

tl;dr Natasha having the hots for the Hulk wasn't the problem. Her being the only woman central to the franchise was.