Sexual orientation doesn't have anything to do with relationship type or status. I'm bi, in an open marriage, but haven't ventured outside my rel w/ DH in several years. Mostly because I was in a stage that was as much "nonsexual" as anything else, focused on the baby. With my libido waking up, I've still been too busy for "outside pursuits", as has DH. So I am monogamous and in a hetero relationship by default, not because that's what my relationship boundaries or orientation actually are.
I went through a period where I had to "come out" a second time as bi, because I came out as Lesbian when I was 16 then joined the Navy and noticed the readily available opposite sex. Now that I'm older and wiser, I know that how I identify myself is as much a sociopolitical statement as anything else.
If I hadn't gone so baby-crazy in my early 20's, and hadn't been in an environment where the male-to-female ratio was 10:1, I might never have really admitted to myself that "boys on the side" were possible relationship centerpieces.
And while I identify as bi, I don't think that shuts me off to attraction only to the 2 standard genders. I had a huge crush on a MTM transexual woman when I was 16 (right before I came out as a lesbian, lol).
One of these days, I hope to wade back into the dating pool. I've never been in a relationship with a woman, yet. Someday I hope that changes.