Am I just at fault?

[Legion]

It doesn't matter who's more wrong.

At the end of the day you're responsible for your actions. If your concerned about doing the right thing you'll tell this girl to make her choice or miss out.
This isn't just to benefit the other bloke or your conscience either. If she thinks its okay to have multiples guys on the go, and is hesitant to leave the guy she's with to be with you, then she isn't going to be a very trustworthy or loyal partner is she?

I don't think the OP cares about how good a partner she is, he is wanting sex and she is happy to have it with him. The OP makes no mention of him wanting more than that. His question is about how right or wrong it is to sleep with another persons partner without their knowledge.

Like you said, he is responsible for his own actions, so he can't claim to not be at fault when he is sleeping with a co-worker who he knows is in a relationship already.

 

[Dead_Man]

If you didn't know she had a boyfriend at first, then no. Now that you know, I'd say that your just as much in the fault as she it.