I had sex with my coworker many times during the last 3 years.?

As long as you are upfront, open and don't lie, I see nothing wrong with what you are doing. Just make sure she doesn't forget that you don't intend to take it any farther.

She has the right to refuse you access to her body so it isn't selfish if you approach her and she openly agrees with your request. Remember, both of you are receiving pleasure from your trysts so there is no need for feeling guilt as long as it's consensual.

You are a selfish cad. Leave the girl alone. She deserves better than to have somebody take advantage of her as you are doing. Grow up and control your animal desires.

I think it would be more selfish to lead her along with the hope of a relationship. If you have been up front with your motives, then you can have a clear conscience. The only caveat is if she distorts things and has a "oh, he is just saying that" mentality.

You're not pretending you want anything else as sex, she knows you only want sex and so if she chooses to have sex with you, then that's what she's choosing for. Even if she's involving her feelings, she is aware you don't anything else so any hurt she feels as a result is on her. She could walk away. So it should not be a big deal, if it's also selfish, well that's another matter.

If you know the woman is really in love with you and therefor can't refuse you when you ask for sex, then you should stop having sex with her. It would be selfish to continue to have sex with her if you know she wants a relationship or another sort of commitment. If she doesn't bring this up or doesn't ask you for it, then you're good.

What two consenting adults do in their private time is up to them. Office sex frequently leads to problems (and frequently means that the people involved lose both the partner and the job) so I would avoid that but that's me. You have to do what you think is right. At least you are being honest about it, to me that is the most important part of the puzzle.