It takes a wedding for her to question everything that forms the fabric of her current life: her shrinking feelings for Sherlock, her growing feelings for Greg and her tricky feelings for Tom. She meets him among friends and moves in with him within the course of three months and another month later he ask her to marry him. When she introduces him to her friends–well, perhaps "flaunt" is a more appropriate word–things already start to go downhill. They argue a lot more often and a lot of those arguments tend to consist of how much more time she spends with her friends than she does with him.
They have another argument after the wedding. She expects it to be about the feelings she may or may not still have for Sherlock, but to her surprise it has more to do with how much closer she was with Greg than she was with her own fiancé. It's true, she did have some good conversations with Greg before and after the ceremony, but she swears it was all platonic, nothing more. They were rather close at the reception, though, and there were a few glances.
Greg, at least, shares a smile with her. All Tom does is frown and cross his arms. When she notices this it becomes clearer to her that she doesn't love Tom as much as she thought she did.
"You okay?"
Looking up from her thoughts, Molly finds Greg sitting next to her as the bride and groom share another dance. She nods and explains her troubles with a shrug. He responds with a smile and offers a dance to get her mind off things, which she accepts.
She feels warmer in Greg's arms than she's ever felt in Tom's.