I am sleeping with my friends girlfriend

I have known them seperately for about 7 years but three years ago they got together. I have to admit i was a little hurt as i had always had strong feelings for her. He isnt a close friend of mine, more a friend of a friend
Me and his gf have always been really close but recently we have been getting even closer. Eventually we slept together and have been for the past 6 months. She says she wont leave her bf because she loves hims but says she loves me too and has no guilt about having sex with me. Its a huge secret that only on other person know about.
Is this Wrong?