The objection is always the same: planning ruins spontaneity. Worth asking how much spontaneity there actually is in a week with jobs, children and two tired people.
You are not scheduling sex
You are protecting time in which it becomes possible. What happens in that time is still open.
Anticipation is the point
Knowing something is coming on Friday changes Wednesday. That is not a workaround for desire, it is how a lot of desire actually works.
It removes the nightly negotiation
Couples who schedule often report the relief is less about the sex and more about no longer wondering, every single evening, whether tonight is the night.
Keep it losable
A plan that becomes an obligation breeds resentment fast. If either of you is not up for it, it moves. That has to be genuinely true.