It's NSFW logic, anything can happen. But to answer your question legitimately, it could be like the characters from Madness Combat, their skin completely covers their mouth. Sanford, Deimos, Jebus and the zombies are the only characters that are seen without their skin covering their mouth. Hank, Tricky and the Auditor are the only characters besides Sanford and Deimos that have bought some kind of drink or food and have eaten it/drank it. So following that logic, I'm assuming mouthless characters can't suck dick unless you rip their skin off or they can just magically remove the skin from their mouth.
GrapheneKid
Where there's a will, there's a way my friend
Thenachowithshame
Truly inspiring.