Yes. She is seeing you behind his back and it makes her worst. But good news, she likes you  if  she is willing to meet up with you. They are not married so go for the kill and be honest. Tell her to choose you and leave him. Then when you are finally  happy together you will always wonder if she is seeing someone  behind your back Vicious  circle  of  life.

the person above me is correct, you dont love her because if you did you wouldnt want to share her with someone else. you wouldnt want a secret affair. You want a secret affair because you want zero responsibility in the relationship and you want just sex thats it. 

Don't do it. For yourself in the long run. The earlier you start healing, the better for you to move on or even atract her, alone, again in future with the better you. But you will be different then.

I've been there. Ive thought as long as I can hold them and be close, I have a chance and that they will choose me. I loved them and they loved me too. I wasn't chosen. And all I did was hurt myself. I felt like I was back-up/plan b. I was just someone who made them feel better while I hurt trying to love them. And all they did was put themselves back into a situation wherethe guy hurt her.

So just let go now. It's not worth the pain later.