Sex drive up, semen amount down. Related?

No. Both of these factors are not related to each other. They are controlled by different parts of the genital tract and different hormones.
More sex->less semen. Volume of semen production tends to be constant thus more frequent sex episodes will tend to reduce volume of semen each time. Increased sex drive is great, reduced semen volume should be of little consequence unless you are trying to impregnate. Best to reduce frequency of sex to once in about 3 days if you are trying to have a baby.