You lose some spontaneity for doing things solo, because you need to check with your partner to make sure you don’t already have plans.

This isn’t really inherent to relationships unless you count “friendships”, too.

Not all relationships are deeply entangled. I have a partner that makes their own plans without checking with me. We use a calendar to keep track of like “we’re doing such and such Friday” so we don’t double book or forget, but that kind of “loss of spontaneity” would happen if it was “trivia night with the guys” or “dinner with friend”