For the record I'm agreeing with Sandra, but to answer your question, forget about the work out buddy. You can try it, but odds are he look at you as being good for a booty call and not much else.

Definitely be straight forward with your co-worker that you did sleep with. Don't sugar coat it, answer his questions, and don't go there again. Things will be at the very least awkward for a while at work, and that depends on how he takes that.

Hopefully this is lesson learned to be careful about who you sleep with.

I don't understand how you could have sex with a person you don't like. Of course it's going to be weird at work, you've crossed the line. Not only that, now you want to have sex with his buddy. Learn to keep your panties on unless you don't mind getting a bad rep. These things have a way of spreading.