I think most of it was because Sharon was supposed to have a much bigger role in TWS along with probably more romantic development until they changed it to give Black Widow more screentime since Scarjo got really huge. Even though I think the second was a better idea IMO. They should've cut their losses instead of playing a bad game of catchup just so they could say they stuck to the comics, especially with all the Civil War stuff also going on in the background.
You know you messed up when even the normies and shippers who prefer hetero pairings, the same people who were fine with Steve and the random girl from The Avengers, also saw it as creepy and rushed. IMO if they absolutely had to officially pair Steve with a girl and couldn't keep Peggy around then Natasha would've been just fine. Even if it comes at the expense of her great "just friends" relationship it's still way more natural than the Hulk, and they did get together in the comics in some continuities.