Hippety ...
I totally disapprove of what you're doing.
But ... after that smack on the wrist ... I will always stand by a pal (even an online pal).
So, putting my disapproval to one side, and opening a metaphorical bottle of wine together ...
I don't think mistresses conform to any physical type.
They represent excitement. The excitement that you felt when you first fell in love with your long term partner.
And novelty.
It's the thrill of something that you really shouldn't have.
One of my pals has a husband whom I could seriously go with. (I'm not going to). My pal knows, and she takes it as a compliment that I fancy the pants off her husband. The fact that he is out of bounds makes him all the more attractive.
A mistress is the same. She is, technically, off limits ... and that makes her all the more desirable.