The main role of seminal fluid in human reproduction is to provide a medium that nourishes and transports sperm during ejaculation. Composed of secretions from various glands, including the prostate and seminal vesicles, seminal fluid helps protect sperm from the acidic environment of the female reproductive tract, enhancing their chances of survival. Additionally, it contains nutrients and enzymes that facilitate sperm motility and viability, playing a critical role in successful fertilization.