Well first of all I didn't think Black Widow was hitting on him,for half the movie he didn't trust her and the other half they seemed more like friends then anything else.

I also think that Cap is still getting his head around the fact that so many years have passed,so he just isn't ready to move on. it's been over 70 years for everyone else but it has only been a year or so for him. I don't think in this movie he had yet reached the point where he was ready to move on from his girlfriend(I can't believe I am blanking on her name right now,but that is what happens when you sleep for a bout two hours in three days)from the 1940s. Remember he was a very shy guy and I think part of him still is and she was the first woman he had a real relationship with so i think part of his arc in this movie was getting to the point where he could move on and that wasn't going to happen with BW. Part of the problem was that his job makes it easy for him to hide from his personal problems and getting together with someone in the same line of work wouldn't fix that,if anything it would make the problem worse. Also didn't Avengers kind of hint that BW and HE were kind of an item? To someone of Cap's generation getting together with BW would kind of be like stealing a team member's girl and I can't see him doing that.

So for those reasons it really didn't bother me that he didn't get together with BW.