Slaves nowadays usually refers to either American-style chattel slavery or Roman-style slavery, both of which were systems much different than Serfdom.
But yes serfs will likely have built it, or were involved with the build under direction of hired stone masons, on order of a noble and with resources a noble paid for, under the general societal rule that serfs were to spend a certain amount of days a year working on infrastructure stuff as part of their taxes. For more details you’d have to dig into the law at that time at that particular place but those kinds of arrangements were incredibly common in Europe in the middle ages. You tended to be able to buy yourself out of having to work, and also pay your way in silver or gold out of paying in grain, livestock, etc.
Overpriced pornstar-branded fleshlights aren’t the only masturbators in town and it’s not like there’s no life-like dildos either. The long and short of it is that the mechanics of a lubed hole are superior to that of a hand and you’re not looking at the entry point all the time so noone actually cares what they look like.
Yes, the branded stuff does exist, humans, male female, doesn’t matter, enter parasocial relationships news at 11. Can you imagine how well Justin Bieber dildos would sell.