Tony knew it was a bad idea to start sleeping with Steve. It could mess up team dynamics, make things even more awkward between them in the future, or just plain get in the way of their trying to save the world. Tony foresaw all of the above but not the advent of feelings, and at the most inopportune moment.
Steve wakes up in the 21st Century. He doesn't think much of it, and it's dubious about him. He meets a Norse God, joins a superhero team, and feels terribly awkward about the whole monument at Arlington he's rendered obsolete by not being dead. Meanwhile, Tony is trying to make his mark on history by being the man who finally drove Nick Fury over the edge.
Less than a year ago Tony’s biggest problem with Steve was figuring out how they could coexist in the same team without throttling each other. Now Tony’s biggest problem with Steve is making sure that the guy doesn’t fall in love with him.