I do not see how you are drawing the conclusion that Lord Arnaulds wife was in anyway in love with Matty. they way I see it is this. Husband (as leeneia points out about 19 nad clearly prefers to spend the Holday off chassing wild life instead of staying home with the misses and showing her a bit of wild life) has gone out after a bit of a row. Mrs Arnauld goes to church and there she spies youg Matty Groves (played by a young Johnny Depp please) walking in the crowd. No mention of any previous just a simple case of husband gone to play cowboys and Indians I am going to shag Johnny Depp. Matty is not even sure who this women is, he says he won't go because he believes her to be Lord Arnauld wife, from the rings that she is wearing, so it would seem that there has been no previous.
Meanwhile the serving man who was probally counting on taking advantage of his masters absents goes running off and grasses to Lord arnauld, who has not caught anything and is beginning to wish he had staded home.
Home he goes Matty some what repleat from over exhaustive physical activities doesn't stand a chance, having killed the poor lad, LA is sure that Mrs A is going to think he is really clever so asks her the question in question. She is well miffed as she has just found a better shag that the grassing serving lad and simply to spit her husband says that Matty was the better shag... silly girl.