Slaves are prisoners from Utopian wars, but the children of slaves are not born into slavery. Most are Utopian citizens being punished.
A few slaves are actually volunteers, who were being treated really badly in their own country. These guys get the nicest treatment (but they're still slaves...)
Sick people are taken very good care of and the Utopians believe in euthanasia (the killing of someone terminally ill, with their consent, to spare them from chronic pain).
Women can't marry until they are 18, men until they are 22. Oh, and pre-marital sex is absolutely not allowed. They enforce this quite harshly because they believe that no one would agree to marry otherwise. (Hythloday doesn't seem to have a very happy view of marriage in general).
So, how do they marry? This part is pretty weird. After two Utopians are engaged, they see each other naked once before they have to marry. This way, the Utopians claim, each partner can get an accurate sense of their fiancé/fiancée's physical appearance before they commit 100%. (Possibly helpful to remember here how much more clothing they would have been wearing back in the day compared to the present).
Utopia is the only country in that area where they practice monogamy (only being married to one person) and they do allow for divorce if there's any cheating going on or things are really, really miserable. However, this can't just be some natural flaw in the person or old age, because that is just not cool; you need to stick marriage out if that's the case.
Cheaters are forced into slavery and if they are caught again, they are executed.
Speaking of punishing, there aren't actually fixed punishments in Utopia. Every particular situation is considered by the senate. They find slavery a useful punishment because everyone can see them and be like "nope, that is not for me!"
Seduction, even attempting to seduce someone, is also punished harshly.
Fools (remember them? We encountered them earlier in a story Raphael was telling. See "Symbols, Images, and Allegory" for more) are very popular in Utopia. Utopians like everyone to have a sense of humor and not take themselves too seriously. Shmoop would fit in quite nicely.
However, making fun of people with disabilities is hugely looked down on.
Everyone is expected to take care of their appearance, but they don't believe in make-up.
To encourage good behavior, they advertise honors in the main square so everyone can be impressed.
You can't campaign for political office and officials aren't arrogant and don't go around wearing special clothing or badges.
There are only a few laws in Utopia to avoid any legal confusion or the need for lawyers. Everyone can be a legal expert and defend themselves when the legal system is nice and simple.
Some of the surrounding countries admire the Utopian government so much, they have their own leaders and officials sent over from Utopia. These Utopian officials are all much more honest than your average official since they don't care about money and as result can't be bribed.
They also don't have any use for treaties; everyone just breaks them anyway. Although treaties are popular in Europe for both political and religious reasons, Utopians think that they don't actually secure an agreement well.
Also, they think treaties assume that people will cheat each other. They'd rather assume people are trustworthy until proven wrong. That's optimistic.