Having this meant the possibility of having to live without it when it ended and Steve knew that's what had kept him from ever taking the plunge before. But knowing now that he'd watched Tony go off and have a family and live in a world where they never saw each other, it was just too much. Steve had missed out on so much and somehow it was Tony who made him brave.
Steve could relate, kissing Tony made the world condense down. He wasn't thinking about Natasha, or the future, or Thanos. For the first time there was nothing except his head full of Tony, which only intensified as he moved into his lap.
His name on the other man's lips had never sounded quite like that and a shiver rippled through him as he grasped his waist more tightly. It was more difficult to think about taking things slowly with him there and Steve stroked his hand slowly up his back.