The gap between what people think about and what they have ever said out loud is enormous. Closing it a little is one of the higher-return things a couple can do.
Start smaller than you want to
Lead with something mild. You are testing how it is received as much as sharing it, and there is no prize for going first with the biggest thing.
Write it if saying it is hard
A message is easier to send than a sentence is to say, and it gives the other person time to react privately rather than performing a reaction on the spot.
Say what you want from telling them
"I am not asking to do this, I just wanted you to know" prevents the most common misunderstanding before it starts.
React well even if you have to fake it briefly
The first reaction is remembered permanently. If you need time, say you need time — anything is better than visible disgust.