One person wants sex more often.
The other wants it less.
That difference can become one of the most emotionally loaded parts of a relationship.
The higher-desire partner may feel unwanted.
The lower-desire partner may feel pressured.
Soon sex is carrying far more than pleasure.
It is carrying reassurance, rejection, obligation, shame and fear.
Difference does not automatically mean dysfunction
Couples rarely have identical desire at all times.
Work changes it.