This article is from the BiSexuality FAQ, by Jon Harley email@example.com with numerous contributions by others.
Yes, some are. It depends on the individual. It's like asking "Can a straight
person be monogamous?" Some bisexuals are monogamous, and some aren't.
Monogamy is the socially sanctioned option with respect to relationships, but
then so is heterosexuality. It should be up to every individual, of any
sexuality, to choose the lifestyle which is right for them.