The use of the language that you refer to draws attention to one of the principal themes of this excellent tale, which is that of honour and how we define it. In a sense, what creates the conflict in this fascinating story is the way that different characters think of honour and how they define an honourable person. In the medieval mindset, let us remember, a gentleman's honour was based in loyalty, bravery, truthfulness and keeping your word. For women, on the other hand, honour was all about chastity. Note how this creates massive conflict between Dorgen and Arveragus when she tells him of the "trap" into which she has fallen after determining to kill herself:
But you shall keep your word, I swear you shall!
As I pray God have mercy upon me,
The love that I bear to you is so great
I'd rather stick a dagger through my heart
Than have you fail your promise or break faith.
Honour's the dearest thing in a man's keeping.
Truthfulness and keeping the promises that you make is therefore placed as being more important in terms of honour than chastity is, and even though it will lead to Arveragus being cuckolded, he insists that his wife maintains her honour by keeping her word. The use of contractual language therefore underscores the discussion about the word "honour" and what the definition of "honour" actually is.