This article is from the Polyamory FAQ, by email@example.com (H. Wilper) with numerous contributions by others.
No. There are many polyamorous people who are also bisexual, and
many who are monosexual (i.e. relating only to one gender as
potential or actual sexual/romantic partners; straight or
gay/lesbian). There are also lots of folks who don't do sexual
preference/orientation labels at all. One doesn't always know
until one asks, as with so many other things. Avoiding
assumptions is usually worth the exercise.