You need to provide more context to get a real answer. Is the husband a friend of yours?
Certainly, an advance toward a married woman amounts to a challenge to her marriage.
The question is, is that challenge appropriate? (It could be a terrible marriage.) And even if the challenge isn't appropriate, isn't it the woman's responsibility to reject you?
If the husband is a friend of yours, then the situation is more clear, because it involves dishonesty to a friend if you keep your intentions hidden. If the husband is your friend, in general you should tell him, and let him judge you appropriately.
You could lose a friend as a result of your actions, but being dishonest with a friend is much worse. You are much better off losing a friend than keeping a friend under false pretenses.
Most importantly, there is no intrinsic requirement to refrain from challenging a marriage. One should refrain from challenging a friend's marriage, and one must recognize the risks of challenging a stranger's marriage, but marriage is not, as such, untouchable.