I know this is a very short reply (apologising to the mods) but the answer can be given in a TL;DR manner.
Yes, the husbands generally knew. In fact, the entire court most probably knew.
Why did they not care ?
That has multiple reasons :
she was mistress to the KING, if you liked it or not, that didn't matter.
the husbands often got some rewards, like lands or titles to keep them happy.
the conception that we have today of marriage, is not at all the 18th century conception of marriage. Most of these men were married to somebody they didn't love, and most probably had mistresses themselves. So they didn't really care.
Some did.
The husband of one of the principal mistresses of Louis XIV, Mme. de Montespan, objected to his wife's liaison with the king. At one point, he installed a set of horns (the symbol of the cuckold) on his carriage and drove about for all to see. Louis XIV eventually found this to be tiresome, and M. de Montespan ended up in the Bastille:
...in order that nothing should be wanting to complete the licence of this life, M. de Montespan was sent to the Bastille; then banished to Guienne, and his wife was appointed superintendent of the Queen's household.
Sources: Saint-Simon, Memoirs of Louis XIV (Chapter LXXV) and Nancy Mitford's The Sun King