Somehow I think dear old Tony as stable and reliable but Charles as a smooth-talking Don Juan. About who loves Lady Mary more, it definitely is Tony. Perhaps if Tony played hard to get, Lady Mary would be head over heels trying to win him over. She just loves the excitement and is taking the bull by its horns, whether or not it hurts people (poor Edith!)
Now that Lady Mary slept with him, it seems like she tossed him out after a bad sex session, but that isn't the case. Yet I guess it makes things clearer that once you had tasted the forbidden fruit, it is no longer desirable.
I would like to see her with Charles Blake as he seems to match her in wit and he is not too bad-looking himself. Things would get pretty routine with Tony, but not with Charles. But poor Tony, what is to become of him?