from season 3, episode 7 (Nutmeg)

Sherlock:

Your romantic inclinations are not a flaw to be corrected, they're a trait to be accepted. I know you, Watson. I know you'll never be happy within the confines of a quote-unquote traditional relationship, and I said what I said because it pains me to see you try to fit into one simply because it is the default mode of polite society.

Joan:

Well, there's no reason to feel pain, because I'm happy with Andrew.

Sherlock:

But would you be happier without him? Alternatively, with him as an occasional sex partner and confidant, or, with him when he's within the States and free to pursue other interests when he's not? There are any number of any possible arrangements, all you need to do is find one that is true to your nature.