"Sorry," she says when he looks uncomfortable at the mention of Tom's name. She blushes a bit in embarrassment. "I mean...sort of. I guess. Sorry that I brought him up right now. But...he was my...boyfriend for...a while, so he's probably going to come up. Occasionally."

It's just a fact that she was engaged to the man. She can't change that, just like she can't change having dated Jim or the other blokes she'd dated before that. They were all part of her history and who she is. Sherlock is her present and (she hopes) her future.