Why didn't Spider-man join the roster of X-Men? EDIT-( Until I asked this question, I had always thought he was a mutant.)
Although the X-Men have occasionally taken on non-mutant members, Spider-man hasn't shown interest in joining. Until recently, Spider-man had always been an independent adventurer and did not join up with other super-heroes in teams.
In the current continuity he is both a member of the Avengers and the FF. This is a new role for Peter Parker, who has matured since his youthful independent days. So it's possible, I suppose, that he could someday join an X-Men franchise - but since he's booked up with Avengers and FF that currently seems unlikely.
Actually, Spider-Man has a history of teaming up with other heroes. Though not officially canon, the "Spider Friends" have had several adventures together, and his friendship with both Ice Man and Fire Star was shown to be deep even beyond their college years. (Angelica even helped him while she was a New Warrior in the 1993 Maximum Carnage arc.) He's worked with the Secret Defenders, served the Fantastic Four and S.H.E.I.L.D. in various capacities (most notably in the Ultimate Spider-Man world) and has been a "Reserve Avenger" since at least the 1990s.
I can't remember the story, but there was a cross-over in the Spider books where he, Punisher, Moon Knight, Night Thrasher and somebody else (Dark Hawk, I think) had to team up to rescue Nova (Thrasher's fellow New Warrior at the time) from this massive crime organization which had made some super-human cyborg operatives and sought to take it to a next step by augmenting super-humans. In that book, both he and Moon Knight were shown to be "reserve Avengers" in possession of a telecommunication card, which was issued to all Avengers members.
As to why he never "joined" the X-Men, well... Spider-Man, much like Hulk, Sasquatch, Scorpion, the Lizard, Spider-Woman (both versions) and some other Marvel non-mutant characters are what's known as "human mutates." Unlike the X-Men, who were all born with an X-gene which had them develop their abilities naturally, Mutates are humans who via some kind of accident develop their abilities from an outside source. It can be chemical, technical, energy-based, or whatever, but while it does alter their genetic structure (or in rare cases, activate a dormant x-gene) they weren't born with active genes, which is the hallmark of a mutant.
On a bigger scale, due to his non-mutant status, it could be argued that Peter wouldn't be able to "identify" with the mutant condition the same way, and he may have had a moral issue about that. It's the difference between dating someone of a different race and thus suffering some effects of racism, versus being a person of a different race and receiving the negative aspects of it full bore.
Lastly, a good portion of the X-Men joined as adolescents or young adults; Peter had already been a practicing super-hero for years before their para-military vigilante organization went public. Though he does work well with teams, Pete's sense of commitment is another thing; he has always been about defending New York, being the "Friendly neighborhood Spider-Man," and the X-Men often times face greater world threats. I don't think he'd easily give up doing that anymore than Daredevil would look to expand his reach permanently beyond Hell's Kitchen, especially considering his responsibilities to both Mary Jane, the Bugle and Aunt May.
Spider-man initially was not thought of as Mutant because it was radioactivity from the spider bite that was thought to have caused him to change into the Spider-man we know. He really did actually mutate but Professor X did not want to go down another path of mutations unless they were actually born with the X-Gene. So Peter was never really born with this gene so technically not a mutant.