It's Will who finally approaches him about it and asks a question instead of just insisting they must be dating: "Are you really not dating Robin, because it really seems like you are and I don't understand."
"C'mere, kid," Steve shifts so that Will can settle in next to him before continuing—if there's one of the kids who he thinks might understand, even just a little, it's this one.
"No, I'm not dating Robin, but everyone thinks I am because they can see that we're, like, life partners and all of that stuff that usually goes along with dating and we love each other a lot but would never want to date each other; Robin and I will always be together, so a lot of people are going to be confused for a long time, but as long as we have each other it will all be fine," he tries to explain and knows he's not being anywhere near as articulate as he would like to be, but Will nods and seems to be understanding some and that's good enough for Steve.