Brotherhood for A Reason
The motorcycle gangs are touchy about the topic of brotherhood, and they set tons of ground rules regarding this specific point. In fact, one of the reasons they have strict rules about who joins and who doesn’t is because once a man joins the club, he has to put the brotherhood of the members first.
That is also probably one of the reasons they don’t allow women to the full members, even though women play a crucial part in their everyday dealings. For married men to join the club, their partners have to have the same level of commitment to the club.