Should I wear a cup for sparring? I've been getting interested in Karate sparring, but I'm still murky on some things.
Absolutely. There's no reason not to; I've never had it interfere with kicking. I suppose some brands could be disagreeable to your particular anatomy, e.g., I had one once which was too "shallow", so I suspect that if I got kicked there it would have given shock waves through the entire package. If you try one and it's uncomfortable, just buy a different brand.
Always wear a hard athletic cup before walking onto the mat. My system is boxer briefs with pouch as base layer, regular 3” waistband jockstrap as next layer. Then insert banana style hard cup between jock and base layer. Make sure the cup is in place with the jock pouch completely covering the cup. Press cup to your groin making sure scrotum is inside cup.
Stay away from flex cups, soft cups and old fashioned triangle flat cups, they are not designed for upward kicks to the groin.
Sparing at less than 30% strength, you will be well protected from most direct strikes.
No one should want to hurt training partner and a good training partner should protect themselves.