Most people find it easier to do almost anything than to say what they want out loud. The fear is not really embarrassment — it is that wanting something specific will land as a complaint about everything that came before.
Pick the wrong moment on purpose
Straight after sex is the worst time: everything said then sounds like a verdict. A walk, a drive, anywhere side by side rather than face to face, takes most of the charge out of it.
Ask for an addition, not a correction
"I would love it if we tried…" lands completely differently from "you never…". Same information, and only one of them makes the other person defensive.
Trade, do not interrogate
Offer something of your own first. A conversation where one person is asked to expose themselves and the other just listens is not a conversation, it is an examination.
Accept a no gracefully
If you want honesty, it has to be safe to say no to you. The first time a no is met with sulking is the last time you get a real answer.