In victorian england/era, a man could not divorce his wife if she was insane.

Mr. Rochester is also particularly important to Jane because he provides her with the unconditional love and sense of family that she has never experienced before. The revelation of his marriage to the insane Bertha Mason demonstrates that Jane possesses the moral and ethical superiority in the relationship. Jane rejects his marriage proposal after she learns of Bertha, not only because she feels it would flout the law, but perhaps because Bertha's marriage is a cautionary symbol of Victorian marriage: despite Mr. Rochester's best intentions and Jane's equal intellectual standing, he may still end up imprisoning Jane in his own way through matrimony, just as he has imprisoned Bertha. Given this fact I think that Rochester was not right to ask Jane for marriage because Jane is well aware of all these layers at play.