This might be pointless considering the 'Use your brain' and 'fake conspiracy' bull, but...
She was a woman who had made a connection with Banner and wanted more from him seeing them both as something of either outcasts or lost souls in their group. She knew Hawkeye was married with kids so he was off limits. Captain America is a Boy Scout and not only seems uninterested in relationships likely they'd butt-heads a lot in any relationship given their vastly different personalities. Tony, well, he's Tony. Not to mention he's with Pepper. Thor is a god and, presumably, already in something of a relationship. So who else in their little group is there to connect with? Banner. And given that she has the "ability" to calm The Hulk back into Banner they likely spent quite a bit of time with one another to develop the technique and that likely required something of an emotional connection.
Great observation. How is anyone not getting that?
How is that a great observation? Natasha doesn't have any other options, so she has to get together with Banner? Why?
And why does the connection have to be romantic? Why, if there has to be a new romance, does it have to be Natasha? (Well that one I know why, it's because she's the only main female character)
She's smart, not stupid, She knew she was out-manned and out-gunned in Ultron's captivity and any attempt at escape would have been suicide.
All true, but to please certain members of the fake controversy / "offended" set, she should have been invincible--incapable of capture, and certainly not needing the help of a man.
Considering how popular CA:TWS is in terms of her character, where she's shot and bleeding, has the rug completely pulled out from under her multiple times, and often has to be saved by Steve, I'm not sure where you get this idea that Nat should be 'invincible'. That's not the problem.
The problem I have with the kidnapping is that, on top of everything else, Natasha is now thrust into the role of damsel in distress, waiting on her boyfriend to save her. That's literally the context that this bit of plot was built around. Sure, she's able to get a message out using the bits of tech left in her cell (which, why did Ultron leave them in there again?) to help save the day, but how is that functionally different from Rapunzel letting down her hair? Banner is still the one to come along and rescue her.
Just to be clear, damsels in distress who help save themselves are fine. So is a character being a mother figure, or a woman in a relationship. It's when you suddenly make your one main female character, regardless of what we've known about her before, a mother, lover and damsel all in one film, while including a plot point about how she can't have kids and bringing up this plot point in a scene where she calls herself a monster, that things get too much.