Anywhere from about 40 to 60% of ejaculate comes from the seminal vesicles. This fraction is basic. About 15 to 20% of ejaculate comes from the prostate, and is acidic. The greater percentage of basic fluid counteracts the acidity of the prostatic fraction. This is normal physiology. The normal pH range for semen is 7.2 to 7.8 (basic.)

About the only thing that will cause acid semen is blockage or other pathology of the seminal vesicles.

The pH of the semen is very unlikely to be affected by diet.