The gossip columns and tabloids speculate endlessly about Tony's sex life. The endless string of dates inspired one of two reactions: either he's easily bored and therefore in need of exotic entertainment, or he's wildly sexually adventurous and quickly scares off his bedmates. It doesn't matter than Tony had only actually slept with a handful of people. Okay, maybe two handfuls. The point is, the media is utterly convinced that his bedroom antics could put an adult video store out of business if only there were tapes.
This made the reactions interesting when he and Steve went public with their relationship, because half the media seemed to think that Captain America didn't even know what sexwas. After the first storm of confusion was over, the gossips settled into a series of "stories" which revealed Tony's increasingly bizarre kinks one at a time and predicted, each and every time, that Steve was absolutely, definitely, on the verge of leaving him this time.
But months went by and Steve didn't go anywhere. Finally, some mysterious tabloid threshold was reached and the stories started speculating on what bizarre kinks Steve must have to keep Tony interested for so long.
The truth is, Steve is damn near as vanilla as everyone thinks. He has a more than aesthetic appreciation for certain items out of Tony's wardrobe, he loves dirty talk (even though he has to be on the verge of coming before he can reciprocate), and he's willing to indulge Tony in a little light bondage. That's about it.
But the dictionary definition of "kink" is "bizarre or unconventional sexual preferences." In that sense Steve has one huge kink, a kink that, for Tony, dwarfs all other kinks.
He wants to be with Tony just because it's him. Steve doesn't need anything else.