"Most people didn't really know that you two grew up in competition. Not many left, anyway," he pointed out. "Do the other Savage Snapes approve? They're the only ones I think you have to worry about, and I know what S- my old professor would say."
Given a moment to think, Harry decided he would agree with Sirius -- better two people he left behind find happiness together than continue on alone out of some misguided sense of propriety. He loved Regulus as a friend and held a soft spot in his heart for the way Regulus held him together in a time when he was coming apart. If he found what he needed in Gaius Snape, Harry wanted to see them get to that "long-term" Regulus mentioned.