Falling in love with Captain America's boyfriend is no doubt at the top of the list of Tony's most stupid accomplishments. He knows Bucky would ever choose him above Steve — not when the two supersoldiers have been fighting for seventy years to find their way back to each other. Fact is, even Tony finds that ridiculously romantic and wouldn't dream of interfering. More than anything, he wants Bucky to be happy.
So when Steve and Tony find themselves trapped inside an old HYDRA base during a mission, slowly being gassed to death, the choice to put Steve's life above his own is easy. Captain America is more important than Iron Man, and Bucky Barnes needs Steve Rogers a whole lot more than he would ever need Tony Stark.