> If there are possibly valid reasons, shouldn't we instead discuss those?

That's a matter of opinion, but since you asked me: no. Every discussion of single family zoning should start with its racist origins and go on to consider the negative racial and socioeconomic effects it's still having, and to hell with parking and the other selfish, greedy, petty, short-sighted, and/or misguided reasons for denying folks affordable housing.

We don't need to speculate. There is strong support in the historical record that many cities adopted single family zones for explicitly racist reasons.