Is Semen already present in the male body at all times? OR is it only produced by the testicles during sexual activity?

Semen is a mixture of sperm - which are constantly being produced in the testicles - and various fluids secreted by glands, such as the prostate. The mixture occurs during ejaculation, so semen per se doesn't exist all the time, but the components are always present.

Semen are always there,stored in epididymis. Whereas ovum is produced after 28 days gap in females, more likely 56 days gap per ovary (as two ovaries alternately produce ovum)