One thing that must be remembered in this context is that the members of the Fellowship are all chosen. This is not simply a band of adventurers that comes together by happenstance, it is a select group that has been put together after the Council of Elrond, with the specific purpose of undertaking a perilous quest.

Clearly, in selecting the members for this quest, one consideration must be paramount, which is that of exactly how dedicated the members of the fellowship will be. In this, I am reminded of the reasons why the members of the Night Watch in the Game of Thrones novels are required to remain single and not have children. As Aemon Targaryen puts it to Jon Snow - Love is the enemy of duty. When it comes to the crunch, it is human nature to value our loved ones more than our sense of duty. Which parent would be willing to sacrifice the life of his or her child in order to see the quest through? Even if there are such candidates around, would Elrond have been unwise enough to select a married man or woman to join the fellowship? Why take the risk that the Enemy may use the wife or child of a member of the fellowship to gain some kind of advantage, when it would be simpler to avoid this situation by choosing a single person?

Legolas, Gimli and Boromir have been selected precisely because they have no marital ties. It is possible that they may have been selected to attend the Council by their respective leaders because they are free of these ties. With the hobbits it is a little more complex, but the process is akin to that of natural selection.

Merry and Pippin choose to follow Frodo only because they have no wives. If they had not been single, it is likely that they would have chosen to stay back, like Fatty Bolger. In the case of Sam, it is again a question of being chosen, not by Elrond but by Gandalf.

To summarise the answer to the original question is that there are no members of the Fellowship who have a spouse because Elrond in his wisdom probably judged that to send a man or elf or dwarf who had a wife whom he would have to leave behind would simply place what is already a very difficult quest in even greater danger.

I agree that the Council's members were "chosen", but it seems that the choice was the work of other powers ultimately than any leaders.
Granted, Legolas was sent by Thranduil to Rivendell to explain Gollum's fate, and Boromir's purpose from Denethor was to get the meaning of a dream, but there was no expectation on their part that they would be going off on any sort of quest afterward. Elrond stated that the Council was "called", but not by him, so I think it's clear that the Fellowship was also assembled under the direction of some other mind.
That said, I would agree that an unmarried status might have been a qualifier.

That makes me think that the reason that "gang" formed was because it started from a sort of "bachelors' club". If Merry and/or Pippin were married, they probably wouldn't have hung out with Frodo as much.

Yeah, look at these dandies:

http://th04.deviantart.net/fs70/PRE/...an-d378a2m.jpg
(x)

I think Saruman saw that in the palantir and was thus moved to destroy the Shire. :D

After this, I'm never going to draw hobbits without sideburns again.

Why does the hobbit in the foreground look so much like Ringo?

Well, if we're having Hobbits as Beatles then it's obviously Pippin.

I mean, Frodo = John
Sam = Paul
Merry = George
Pippin = Ringo

Well, it wouldn't have included Sam then, what with him being just a lowly gardener (as well as being interested in marrying Rosie).

Another interesting thing is that the people we know had wives because they have children (Theoden and Denethor) also don't have them - I presume they died.

Havent you heard of the story of Ringo Baggins? He was the drummer in the band 'The Shirelads'. They pretty much didn't ever make it farther than the Green Dragon/Floating Log/Golden Perch bar scene, though they did play at Chetwoodstock, a 3 day music festival north of Bree.

Back on topic.... if you're married, going off on prolonged adventures without your spouse is somewhat detrimental to said marriage. Ask Aldarion and Erendis.