What does a male condom prevent when used correctly?

Using a male condom correctly provides a barrier that prevents sperm from entering the vagina during sexual intercourse. This is a primary function of the condom, as it is designed to catch semen, which contains sperm, thus preventing it from reaching the female reproductive system. This is crucial for preventing pregnancy.

While condoms also help reduce the risk of sexually transmitted infections (STIs), that is not the focus of the selected answer. Additionally, a condom does not interfere with ovulation, as that is a biological process that occurs independently of the use of condoms. Semen leakage could occur if the condom is not used properly, making the prevention of sperm entry the most accurate choice in the context of the question.