Okay so it's not that Steve isn't used to spunky, sexually liberated women as much as he's not used to women who speak to him without a motivation of 'can you hook me up with your friend Bucky' and even after a few months in the City, women like Amy still kind of throw him off a little. He's not sure what he should think of her on a personal level - she's nice to him and she's clearly trying very hard but sometimes he doesn't know how to deal with her.

However, that said, she seems to be really good for Bucky and she makes Bucky happy, which automatically makes Steve like her. Steve isn't a jealous friend, and he really likes that Bucky's making friends because he's noticed that people who make friends tend to stick it out longer than people who do and, well.

He kind of wants Bucky around for the long haul.