He can almost do everything: eat, drink, poop (Luffy asked him when they first met), etc. Brook even says by drinking milk, his broken bones heal faster. So it seems food has effect in his body.
So I was just wondering that after a certain age, all the straw hats will retire and have a family. Will Brook also be able to do so? Can Brook marry a real woman and have real kids (human kids)?
Yes
I could not find any verified references as to which of the two possibilities listed below is correct, but based off the fact that he is non-existent parts can interact with the things around him [relevant AAM question] I am leaning towards the first.
1. Brook just acts like most people when reproducing and via the Yomi Yomi no Mi fruits powers, somehow it all just works. The sound waves in his chest bounce off something, his urine is aimed by something so it seems probable that if he just went through the motions, (whether or not his wife could feel anything) then his non-existent organs would also operate normally.
2. This is to cover all bases, and explain why it would be possible, although admittedly not practical if he was unable to become aroused or physically respond down there. Because Brook has other liquids, it is fair to say that he does have sperm and we know that if nothing is happening, eventually Brook would have a wet dream. I would like to refer to this article about frog underpants that was placed on the frog in a very early experiment to learn about the basics of reproduction where they placed specially made underpants on the male frog, placed him with a female, waited for them to mate and then collected the frogs sperm and analysed it. Well in theory, brook could wear a similar apparatus to collect and preserve his own sperm, which could later be used by his wife to produce a child.
Plus there is always adoption
So I was just wondering that after a certain age, all the straw hats will retire and have a family.
I highly doubt that.
Will Brook also be able to do so? Can Brook marry a real woman and have real kids (human kids)?
The other answers indicated that he somehow seems to function just like a normal human being. However, in my opinion, this only serves comedy. None of his "normal human being" traits had any impact on the story. I highly doubt it would be possible for him to father children, because that would mean an impact on story (even if it is just the aftermath of the manga).
... On the other hand I have no doubt that there is fanfiction about him being able to do so.
In short, his organs are fully operative but they don't exist.
Since you need "that organ" in physical form to have kids, then I assume he can't have kids (his genitalia can't be touched). But he can still pee since he doesn't need to touch his genitalia to pee (like he can have a nosebleed without having his blood circulating his body)
