This is a hard question to answer. 1st to dabeaner, that's harsh!!!
Now, if you haven't told him your pregnant, tell him and see how he reacts. Tell him how you feel, don't be scared as to how he will react. I told a man that I was in love with him once knowing that he wouldn't say it back, but I felt better knowing that he knew how I felt and it was up to him to respond.
If you don't want to tell him, decide whether you want to have a baby by a man that you're not sure loves you and wants to have a child with you. Be sensitive to his situation as well. He is a married man and he will have to choose who he wants to be with. At the same time, you have to be ready if he happens to make the choice to stay with his wife.
I believe it is a woman's choice to have a baby, but if this is your 1st child, the decision will be hard to make. Are you keeping the baby hoping to get the man who helped you make it? Or are you going to have the baby to raise for yourself, with or without his help? Will you be able to live with deciding to get an abortion? The choice is yours.