[Kizzy]

The term 'mistress' is quite out moded to be applied to any modern relationship, mistresses were taken without the consent of the wife and conducted in secret.
I don't think having a long term affair could or ever would add to a marriage, the deception and the divergence of feelings for both women would be detrimental to the marriage and if discovered could end it completely.
Personally I would not like to be the mistress, should the affair last there would be the distinct risk of developing feelings for the man you are sleeping with, added to that the need for a loving family of your own where you happily provide both a happy home life and a fulfilling sex life.
There should be no question of a divergence of roles, if that were the case we could go on forever selecting mates for specific roles... I could have Mr Monday for his bedroom prowess, Mr Tuesday the electrician, Mr Wednesday the gardener...
Hang on, I'm having a rethink....jk.