I am 18 years old and am very involved in weightlifting, so I eat and drink lots of high-protein foods.

I've heard semen contains protein, so by ejaculating, am I losing a lot of protein that I need to build my muscles?

Semen does contain protein, but the amount you ejaculate is negligible compared to the total amount your body takes in. Eat an extra peanut or two and you've more than made up for the protein "loss" from one ejaculation. So no, masturbating will not reduce your body's ability to build muscle.