The former. Ideally into an organism but if there’s enough external stimulation, as one could imagine with certain species of sea anemone, that could also work.
I cannot believe I am replying to this thread, but…how about fungi? Many are rather phallic, but some are invaginated, so to speak. Like trumpet mushrooms. I imagine the texture would be passable.
Have a look.
“Are you looking to find some sort of organism for a human male to masturbate into…”. Answer from the OP was yes. Sheep have been the age old “go-to”.
<insert Scot/shepherd joke here>