I am glad to hear your perspective on this type of situation. I am in a committed (lesbian) relationship with my partner who I feel betrayed by. So I am on the opposite end of this scenario as you are. My partner has lied to me about contact with another lesbian that she met years ago on a dating site when she was still single, among many other lies. She says that they are "just friends" but she has lied to me many times in the past, so I don't know what to think. This friend sent my gf poems she wrote that professed her love to her. My gf didn't hide those, but I did ask her to be respectful to me and back off of those communications. She said she would and had, but months later, I found otherwise. Now, I have no evidence that their conversations went to a sexual or deeper level because she hid the contact and deleted all texts. But because she hid it and lied about it, and that we have a history of trust issues, I will never be able to trust that memory, her, or even me completely. My partner is the most important person that has ever been in my life and I love her immensely. I thought I was the same to her, but feel unimportant and broken because of her actions. And I can honestly say that my heart is shattered maybe irreparably. Although I choose to stay and work things out despite the lies, I will never be able to have the same amazing relationship with my partner as we could have.
I just say this to give you pause before inserting yourself into someone else's relationship. No matter what the woman you are seeing says, she won't tell you the whole story. And the whole story is that a marriage could be destroyed by your actions. Are you prepared to live with that responsibility? If the answer is yes, then there you go. But if you even hesitate, then run the other way...it isn't worth it. Good luck in your choices, I wish you all the best.