|A Young, 18th Century Man's Wedding Portrait|
He was trying to discourage his friend from acquiring a mistress.
Of course, in that same letter, he goes on to suggest that, if the young man cannot be dissuaded, he should choose an older woman, "Because when women cease to be handsome, they study to be good."
I'm sharing this because I find it amusing, but I also think it holds a kernel of truth. Numerous studies have shown that the happiest of men are married men. But also, while beauty is fleeting, a good soul is not.
If you're interested in the full text of the letter, especially his other rather amusing reasons (or insulting depending on your sense of humor - see #5) for choosing older women, here's a link.