Brewing of the polyjuice potion requires a piece of the person- hair, nailclips, or anything else. Normally, it is hair. Grindewald however is not your typical wizard, plus he loves humiliating his victims.
Instead of using Percival's hair, he uses his sperm.
Hence- chained and protesting Percival and Grindewald who enjoys sucking him off or fucking him to make him ejaculate and collect the ingredient.

Bonus if Grindewald keeps Graves's appearance (sorry but that bleached pineapple can kill any boner), so Graves is basically getting sucked off/fucked by himself.