Steve had never considered Tony to be second best, not even close. It was Tony who had got him to take the plunge, to want something more than he wanted to protect himself. But he could understand those insecurities, even if he hoped to be able to reassure him some and put this behind them to focus instead on a future together.
He was right though, Steve had been very guilty of lying by omission, keeping things from him and hurting him because of it. Steve could have continued to be vague and he wanted to, there were reasons, like that he didn't want to discuss something so private, or hurt him worse. But that might end them up somewhere neither wanted to be.
"I kissed him," Steve said after a beat. "I kissed him and he kissed me back and we said goodbye to holding candles for one another that haven't ever really burned." There was a chance Tony was going to be furious and he'd understand it, deserved it, but he hoped to avoid that. "And he's in love with Natasha and I'm spoken for and it's time to just let all of what never was go."
All he could hope was that Tony would understand and that telling him the real truth wouldn't backfire and make everything worse. "There isn't room in this for anyone but us and it was time to let myself have this with you."