I think this isn't the right question to be asking. The question you should be asking "Is it OK for me to be involved with this particular married woman?". Lesbian or not, doesn't matter. Other married women don't matter either.
The answer to this can't really come from people on the Internet you have never met. The answer can only come from the woman and her husband. If both of them answer "Yes", you have a green light, run with it. If either of them says "No" it would definitely not be OK for a variety of reasons. If the husband is not on board you would be creating problems for both of them. If you truly love this woman, do you really want to be a source of tension in her life?
So, to reiterate what Jane Q said, find out where both of them are on this, and don't necessarily take the woman's word for it.