It is easy to hear "I do not want to do that" as "I do not want you". They are entirely different statements, and the confusion between them quietly narrows what couples can discuss.
Name limits when things are calm
The worst time to discover a hard limit is halfway through something. A short conversation in daylight prevents most of the situations people dread.
A limit does not need justifying
"I do not want to" is a complete answer. Demanding reasons turns a boundary into a negotiation, which is exactly what a boundary is not.
Limits move, and that is fine
Trust changes what people are open to. Revisiting occasionally is reasonable; pushing is not.
Respect earns openness
The partner whose limits are respected without argument is the one who eventually says what they actually want.