The trouble is that people expect both. Great relationship, great sex.
My observation is that these things are mutually exclusive. You have either the one or the other.
Maybe one partner may have the illusion that the partnership has both things without knowing that the other partner feels like he or she has only one of those or even neither of those.
It’s certainly not common to have both and am pretty sure the majority of couples only have one out of two. Or even zero out of two, but somehow stick together anyway.
Maybe with a LOT of work and a lot of very experienced guidance improvement on both levels is possible, but whatever is not spontaneous is less enjoyable.