Problem number one, that girl in the opening picture has a ring on her left hand. Perhaps try again with single girls?
Is it unethical to go out with someone that wants out of their marriage? A divorce is costly, takes a lot of time, and the relationship is usually dead long before the papers are finalized. If there's trickery involved, then sure. But it's not quite so clear-cut. Where it gets murky is if you don't have a job, you're stuck -- it's very difficult to leave your marriage without at least attempting to find someone else…
Pragmatically, if she dates you while wearing that ring, chances are she will date another dude while wearing your ring.
If you need to stay on toxic relationship because of money, it is hard to leave, but not because you did not dated on the side. It is because you need to arrange living and job (or shelter). People in toxic relationships tend to be emotionally drained.