Block it...that simple.
Where my son trains and competes the groin is a legal target. You should try and learn how to protect it instead of relying on your opponent
A guy who taught me a lot, a real lot, never used to wear a cup. He's what you would call "a pretty good kicker". Many years ago he was at a point tournament, he was doing a three round demo match with the guy who won the Grand Championship.
He was winning something like 23-7....when the guy threw a kick, not a particularly hard kick, just a kick. But it caught him just so. He was taken to the hospital and had to have a testicle removed. That man was Bill Superfoot Wallace. He never sparred or fought without a cup again. So if you ever meet him, ask his opinion on not wearing a cup. He speaks very candidly about it.
In any dojo I ran you wore a cup to class every day. And every few weeks there were cup checks. (And no, not what you're probably thinking, it was a sound check. Rap your knuckles on your cup.) If you were caught without one you spent the entire class, face down on the floor, doing pushups. The class was ninety minutes. You never forgot one again, I'll tell you that.
You also always had to have a mouth piece in your bag with all your fighting gear, every night, not just for sparring. It was your responsibility, not ours. Our responsibility was to make sure you knew yours.
As to the point someone made about "you don't wear a cup in the street" - school, college, any training, and your parents prepare you for life (hopefully). But as you go through life you won't have your school books with you, or your parents, or your cup, you'll just have your training. All those other things are just part of the preparation.
I have a great photo of Linda Denley from Texas, one of the best woman fighters I've ever trained with. I asked her if she could make it a point to show a particular visiting black belt why he should wear a cup. She told me to "stand by with your camera". So I did.
She made her point with out hurting him. It's a cool photo, I'll dig it up.