I believe that a mother-in-law sees her son or daughter as their "baby" who has been given to someone she does not entirely know, someone whose word will be taken before hers and in some cases she will be the third party to know what is going with her son or daughter, hence the inquisitiveness. Therefore, mothers-in-law clutch on to any opportunity to be an influencing factor in an apparent forbidden situation.
To avoid the interference of mothers-in-law, I believe that at the planning stages of the wedding mothers should not be foremost in the planning process or advisors, but should be required only when needed. When mothers are called on to be the decision-makers, it is an invitation for them to be forthright and to involve themselves in every aspect of the marriage. No son or daughter at this stage should show such low levels of immaturity to allow and yield to mother's directives. If situations like this occurs, them Mencken's words are apt.
Mothers-in-law do not have to be the sin of a marriage. Keep your mother-in-law out of your marital affairs. Involve them only when necessary. If you do this your will be able to welcome the smile of your mother-in-law and no repeat the words of the anonymous person "happiness is seeing your mother-in-law's picture on the back on the milk carton.