Sabrina's pulse flutters wildly at the words, here in this context.

She knows that no matter what else, they do love each other, that it's the sort of love that could be what they temper it into. If they hadn't both wanted this, then Sabrina knows they would have been able to return to the love of friends.

They do want this, though. She can feel it singing in her blood, and she catches Rosie's mouth for another needy kiss first. "I love you too," she tells her, "so much." And then, very helpfully, reaches around and takes care of the zipper, as she has better access.