"It is the most natural state of man, and therefore the state in which you are most likely to find solid happiness.... It is the man and woman united that make the complete human being. Separate, she wants his force of body and strength of reason; he, her softness, sensibility and acute discernment. Together the are more likely to succeed in the world. A single man has not nearly the value he would have in that state of union. He is an incomplete animal. He resembles the odd half of a pair of scissors. If you get a prudent, healthy wife, your industry in your profession, with her good economy, will be a fortune sufficient."
However, recognizing the youthful male attitude and the unlikely event of accepting a rational argument he suggested what sort of mistress one should have.
"In all your amours, you should prefer old women to young ones."
For which he presented 8 reasons.
"1. Because as they have more knowledge of the world and their minds are better stored with observations, their conversation is more improving and more lastingly agreeable.As a final note, Ben Franklin is generally referred to as being a 'ladies man' with the implications that he slept around. This essay certainly makes it seem so. He did seem to have a healthy libido and did have a child, with an woman unknown to history, before he was married, but all accounts seem to show that he was faithful throughout his marriage.
2. Because when women cease to be handsome, they study to be good. To maintain their influence over men, they supply the diminution of beauty by an arguement of utility. They learn to do 1000 services small and great, and are the most tender and useful of all friends when you are sick. Thus they continue amiable. And hence there is hardly such a thing as an old woman who is not a good woman.
3. Because there is no hazard of children, which irregularly produced may be attended with much inconvenience.
4. Because the more experience, they are more prudent and discreet in conducting an intrigue to prevent suspicion. The commerce with them is therefore safer with regard to your reputation. And with regard to theirs, if the affair should happen to be known, considerate people might be inclined to excuse an old woman who would kindly take care of a young man, form his manners by her good counsels, and prevent his ruining his health and fortune among mercenary prostitutes.
5. Because every animal that walks upright, the deficiency of the fluids that fills the muscles appears first in the highest part. The face grows lank and wrinkled, then the neck, the the breast and arms, the lower parts continuing to the last as plump as ever. So that covering all above with a basket, and regarding only what is below the girdle, it is impossible of two woman to know an old one from a young one. And as in the dark all cats are grey, the pleasure of corporal enjoyment with an old woman is at least equal, and frequently superior, every knack being by practice capable of improvement.
6. Because the sin is less. The debauching a virgin may be her ruin, and make her for life unhappy.
7. Because compunction is less. The having made a young girl miserable may give you frequent bitter reflections, none of which can attend making an old woman happy.
8thly and lastly. They are so grateful."
He did seem to maintain his interest in women, at one point maintaining flirtatious written contact with a younger women during his marriage and similarly with other women during his time in France. I don't think you can blame him for that.