Tonks assumed he'd at least had a one-night stand. He was a bloody dragon tamer, for Merlin's sake. Just because he'd never been in a relationship didn't mean he'd never brought someone to bed.
"All right," she said slowly once she'd managed to put her thoughts back together. "Being in a relationship doesn't mean replacing your friends and family. Sometimes it means you might spend a little less time with them, because with a new person in your life and a finite amount of time to go around...well, that goes for any new relationship, really, friendship or otherwise. But I'm mostly spending time with Bill during times of the day when you wouldn't be with him anyway. And as far as the conversation being different..."
She sighed, frustrated that she didn't know how to explain it, and made a vague gesture. "Well, you know. Once you've slept with someone, the conversation's always a little different. Bill and I aren't talking to each other any differently than we have since I was fifteen. Honest. And no matter what happens between him and me, he's still your brother first and foremost, before he was ever my friend or boyfriend"--that was the first time she'd called Bill that since they'd reignited things, and it felt strange on her tongue. Not bad, just different. Bill wasn't a boy anymore, and boyfriend felt a little juvenile--"and I'm still your friend no matter what, okay? If you ever do feel like we're ignoring you in specific ways, just tell us, yeah? And you're always welcome at my flat, day or night. No one's trying to shut you out of anything, I promise. It might feel like that until we all get used to things, but that's perfectly normal."