From "Mr. Monk goes on the Run" when asked what he exchanged with a homeless man in order to obtain a coat.
Adrian Monk: I can't tell you, Natalie. Don't make me tell you.
In "Mr. Monk's Favorite Show", Monk volunteers to bodyguard his childhood idol Christine Rapp, whose tell-all autobiography got her multiple death threats or something like that. Natalie reads it, and it's so bad that she has to tear pages out and still won't let Monk read it, the implication being that either described or outright illustrated are some very very horrible sex acts (she even had some sort of fling with Bob Denver). Later Monk is helping her secure her bedroom, and sees a mirror on her ceiling. He comments that it makes a great security feature, to which she replies something like "That's not what it's for." It's actually for rich people to admire themselves mid-coitus.
Not the first time a "mirror on the ceiling" reference was made. It happened in "Mr. Monk Meets the Playboy," when Monk sees Dexter Larsen's bedroom:
Adrian Monk: Look at that. Theres a mirror above the bed. Why would he need a mirror on the ceiling?
Sharona Fleming: Dont think about that now. Not now.
Mr. Monk Fights City Hall had a Councilwomans love nest. There were cheerleading and French maid outfits which were implied to be part of the Councilwomans sex play (though Monk initially mistook them for signs of her having roommates). Then there was a drawer which Natalie opened and very quickly closed before forbidding Monk to look in. Its left to the audiences imagination what the contents of that drawer may be.