Is this morally wrong

So I've been meeting with a guy for some work and there's a massive attraction between us. It's so clear he clearly stated it after just a while we had been meeting. He's been with a girl for 7 years and has left her once for a summer fling and they got back together just Recently. He told me he feels the connection on every level and wants to have sex with me. I want it too, but I know he's with this girl. I don't know her personally but I know she's a good, humble, kind person. Should I follow my instincts or the girl code? Obviously if I knew her having sex with him would not be an option. Thanks