For me, this question can be divided into two issues:
1. Is it unethical to become involved with someone who is cheating on their partner.
2. Is it wise to be come involved with such a person.
I'll start with the second question, because it's easier. I tend to agree with the others who have said that if they're lying to their spouse about something that big, they're probably inclined to lie about a lot of things. So if you intend to become romantically involved with such a person, you're probably lining yourself up for lies and heartache down the road. So for that reason, I would not allow myself to knowingly fall in love with a cheater.
As for the ethics, I have a hard enough time doing the right thing for myself without being responsible for the decisions of others. It would be against my moral compas to actively pursue someone who's in a monogamous relationship, for fear of actually causing the cheating. But if they were to pursue me, I would not stop them on the basis of their other relationship. I would encourage them to be upfront with their partner and talk to them about polyamory, but I wouldn't make the decision for them.
I do not need a label to define me. Labels are sticky and I hate the glue they leave behind.