That's the lesson decorated matador Paco Urena learned when he stepped into the Vista Alegre bullfighting arena this week at the Semana Grande Fair in Bilbao, Spain ... where he ended up with a crotch full of horn.

OLE!!!!!!

Unclear how badly Urena was injured in the incident -- but it seems like his man-junk survived because there are photos showing Paco proudly showing off the bull's ears as trophies.

As you probably know ... the bulls are mercilessly tortured and killed during the fights. As you can see, Urena was wielding a sword during the fight, with the intention of stabbing the bull through the heart so it can die fast. But, that rarely happens because the bull doesn't want to die!

So, what ends up happening is a bloody, painful, torturous exhibition in front of thousands of screaming blood-thirsty fans ... but hey, TRADITION RIGHT?!?