It absolutely is a result of our biology. Actually ACTING on it is a different matter, which is more of a matter of the nature of your relationship. Some couples are totally fine with open relationships because they aren't threatened by adultery. It really is a matter of perspective, knowing your partner, and knowing their limits and how it will affect them. If you know it will hurt them and you do it anyways, you're being selfish.
I wouldn't say it's inherently right or wrong to cheat on your significant other, but because of how it will affect a person you care about, it should matter to you, and it's a personal choice if it's right or wrong. Personally, I wouldn't cheat because I would find myself to be a terrible person because I know it would hurt the person I love. As someone who has been cheated on, I know this pain and would not want to inflict this on others. This doesn't change the fact that I have met people who I've been attracted to and who have been attracted to me, which brought the physical urge to do something about it - the difference was in my reaction - I cut ties because I knew any kind of action would likely result in all three of us ending up feeling wronged and unhappy.