The pH of a drug is not responsible for its effects on sex drive or sexual function. The way methamphetamine affects sex drive/function is through neurotransmitters, hormones, and blood vessels/pressure. pH of bodily systems can have some effects on sex in the following ways:
1. the pH of the vagina should be slightly acidic - normal vaginal pH is 3.8 to 4.5. If it isn't the proper pH it makes the vagina more vulnerable to infections such as yeast or bacteria.
2. When talking about body pH as a whole it usually means the pH of the body's fluids and tissues, which can be roughly measured by the urine or saliva and should be between 6 to 7.5. If the pH of the body is too acidic for a long period of time, one's sex drive or function could be lowered by affects on energy, blood vessels and perhaps hormones.
The pH of a drug you use is unlikely to affect the pH of your body in a corresponding way. Drugs can affect your pH but it isn't as simple as a drug with a low pH will make your body pH low. In fact, if you look at foods, the pH of the food has nothing to do with how it alters your body pH, in fact it seems to often be the opposite, like an acidic fruit will make your pH more alkaline.
It makes much more sense that a drug's dramatic effects on neurotransmitters, hormones, and blood vessels/pressure would have a much greater effect on sex than any slight effect on pH levels in the body.
Unless you are saying that meth of a certain pH is more likely to be impure, as impurities could alter the effects of the meth.