If you want to teach your child that he or she is worth "more than some side action," then be sure to treat every relationship partner with dignity and as a whole person, so no relationship is ever reduced to just "side action." And, of course, choose partners who treat you well. I think that the way you interact with people has a much bigger impact than the number or configuration of the people in your life.
Also, here's how my friend (who is actually mono) came out as bi to her kid (when he was 4 or 5). "Sometimes boys like girls and sometimes boys like boys. Daddy likes girls. Mommy like boys and girls. You don't have to decide who you like until you're much, much older, and you can always change your mind."