Pretty flimsy excuse that she doesn't have time for it, yet still offers her handussy hand to him. It should take a similar amount of time if they're still gonna do it regardless; heck, he's probably going to finish faster anyway if they use her slussy slime girl bits.
Like her thoughts reveal, the real reason is that she doesn't want any more white stuff down there that everyone can see glorping around. She can't clean that up as easily as wiping her hand, after all.