Why does crystal meth make a person so horny?

Methamphetamine mimics the effects of the sympathetic nervous system. The sympathetic nervous system is subdivision of the Autonomic or "Automatic" nervous system. Whenever you hear "Fight or Flight", they are talking about Sympathetic.

When activated, the Sympathetic Neurons cause a release of specific chemicals responsible for movement, alertness, and sustainence. These chemicals are epinephrine(adrenaline) from the adrenal medulla and norepinephrine (noradrenaline).

More importantly, dopamine (The Pleasure chemical) is released at 6x it's normal amount with meth use, and causes that rush and euphoria. Your pupils dilate giving you a wider field of vision, your heart rate increases, and your heart's contraction force increases as well, allowing you the energy you need to "Fight or Flee".

The emotional seat of the brain is known as the "Limbic System". This system is a series of structures and pathways through which dopamine and other neurons travel. If dopamine hits the nigrostriatal pathway; the effect is usually paranoia, while if it his the mesolimbic pathway, the effect is intense euphoria. Your sex drive is a result of the limbic system. Therefore, if stimulated just right; sex drive will increase.