When Martha arrived early and was going to walk in on these two and their round four, and before Castle panicked and told her to hide in his closet; I love that Beckett was willing to welcome her like that, with just a pillow and a shirt covering her body, look at how she just prepares for it, because it was Martha, and she was family, and yeah, it could be embarrassing, but they could all laugh about later. She was in this with Castle, she wants to love him, and that also includes his mother and daughter.