Why are males' pitch lower than females?
Males usually have deeper voices than females because they have longer vocal folds.

Yes

Short and relaxed vocal folds produce a deep voice, long and tense vocal folds produce a high-pitched voice.

Yes on &quot;relaxed&quot;, no on &quot;short&quot;.
Treating the vocal cords as strings: The resonance frequency is a function of mass (or density), tension and length.
Higher mass -&gt; lower pitch
Higher tension -&gt; higher pitch
Higher length -&gt; lower pitch

Male vocal cords are longer and heavier.