Remus hadn't shagged anyone. But the niggling, doubtful part of him couldn't help but wonder if he might have. If it was indicative of some part of his own nature that was unable to commit to one person. He'd blame the wolf side of himself, but he didn't think that was it. he thought it might be just him.
He loved Tonks, but was that enough? He'd always doubted it. He'd lived through enough to know that while love could be a wonderful thing, in most cases, it simply wasn't enough to bind people together when the worst came. And it was never enough, on its own, to stave off the worst things in life. People in love died. People in love argued, and were hateful. And sometimes, people in love cheated. he thought that made him a worse kind of man, but he'd always suspected that, too.
He was quiet for a moment. "You were never the other woman. And even when we were young, Sirius and I never. . . named what we were, or limited it to just one another, even if that was how it ended up mostly, until it ended. There was. . . a brief window of time where we were happy together. But the majority of what happened between us was frought with a war, and the tension and misery it brought. I hang on to the good memories, but there aren't many, and maybe that makes it more difficult to talk about, and that was part of the reason I hid it from you. I am sorry for that."
"I was never unhappy because of you. I was happier than I've been for a very long time with you. But that didn't make for guarantees, Tonks. It's not getting married, though perhaps we did rush that. I don't know what it is, really. But whatever the reason I was that stupid once, I worry that I can't promise it's not still there. And you deserve someone who knows their own mind, and values you above. . . whatever their own issues are. Which I want to do, but didn't seem to." Remus smiled wanly at her. "I would like to just say yes, and be grateful for the opportunity. But I don't want to hurt you again. So. . . for now I think perhaps we should just. . . try to be friends, and find the best ways to raise our son and get along, and. . . see where things end up, when the war is over and things are clearer. If our paths lead away from each other. . .w ell we'll always have the baby to tie us together as friends and family, if nothing else."
It was a careful, diplomatic answer, and Remus felt that probably made it all the more cowardly. But it was what he felt was right.