They aren’t dating for very long before Steve cottons on to the whole thing. Especially after they’re called out on a mission in the middle of the night and Tony’s almost late to the quinjet because he was so busy watching Steve pull on the fabric before running to get his suit.
Steve, being the little shit he is, starts wearing them off the field. When he does, he makes sure to sit near Tony and cross his ankle over his knee, the pant leg lifting enough to give Tony and eyeful of nylon. He always makes sure to wear them when they have a black tie affair to attend, slipping off his shoe under the table and running his stocking-clad toes up the inside of Tony’s thigh until Tony has to quietly excuse himself and run off to the bathroom for a few minutes, leaving Steve smiling into his drink.
Steve basically has a lot of fun with it. Teasing Tony is one of his favorite pastimes, after all.
It’s just a shame that he doesn’t realize the effect it would have on him, seeing Tony in stockings. But the way his stomach flips and his blood rushes south so fast it leaves him nearly light-headed when he walks into his bedroom and sees Tony checking out his own ass in nothing but nylons, he thinks he might be in trouble.
The way Tony smiles at him when he spots Steve’s expression makes him absolutely certain.