A group of males / females who are into riding Motorcycles, or club where the criteria is riding Motorcycles.
The physical attraction for another based on their 'butch' look, wearing of leather, tattoos, and other 'tough guy' look associated with those who ride motorcycles, as popularized in various 'biker' movies.
[undocumented] An attraction for a more outdoor, rough type partner, where limits and rules are routinely ignored. Though in actuality is can also be classed as a sub niche of the 'leather' or 'bdsm' category.
1% - refers more to what society might call the criminal element in motorcycle clubs. However some claim it is a false moniker, because they are about the lifestyle rather than being weekend riders. Popular 1% clubs are however Hell's Angels, and others.
99% - was a tag given by the National Motorcycle Association in its response to the Hollister Riots, where they said that the portrayal of the gathering was where only about 1% where outlaws, the remaining 99% were decent upstanding citizens.
Motorcycle Clubs or Riders are a loose association of fans of motorcycle riding. They are generally fans of a more individualism lifestyle than most, enjoying the feel of the wind, of being traveling through the country without being confined in a shell.
Most 'gangs' or 'clubs' are more social than criminal, despite the reputation given to 'bikers'. It is more about the freedom and solitary riding experience, than about being outlaws.
Clubs are generally part of a major more national group, and share colors or emblems to highlight their differences. In some instances, it is this attention to colors or emblems that has given rise to the claim that most are criminal in nature, which is incorrect.
Many motorcycle gangs, clubs, actually are more socially conscious and perform many acts of charity, and do not get involved in criminal activity such as drug dealing, or other wanton acts of violence as depicted in many modern era movies.
One or both traditionally wear leather garb, have tattoos and are more muscular in build, than some. There is a 'outlaw' type persona associated with those who are into 'bikers', though the actual sex acts can be as vanilla as any more sedate niche.
Being an actual motorcycle rider is not a criteria, but does add to the mystique of the niche itself.
[undocumented] Biker sex or relations can imply a less structured relationship among its participants, supposedly based on the old movies of bikers, where sex was not always exclusive among two people, but generally shared among the group.
This approach follows the traditional sense of 'brotherhood' that typified actual motorcycle clubs, gangs.
Practice (Associated Acts): Being into more rough sex acts, including bondage but not limited to it. Can include use of restraints, or of extreme sex toys, such as over sized dildos, use of whips, and adornments using chains and such.
Noteworthy: The second oldest Motorcycle Club is located in San Francisco, the USA's gay Mecca. Some attribute that to the niche popularity in the Gay culture, though undocumented.