I'm dating an old co-workers ex boyfriend and I feel guilty
Are you distant from your co-worker?
No, I'm pretty close to my co-worker. I don't think it's appropriate though, but I do like the guy who I'm dating.
As long as the relationship doesn't affect your work, your co-worker should have no say in who you date. You could talk to them about it you  want to alleviate your guilt, but you are not obligated to do so.
Communicating with people should be a norm just like I was talking to you. It should not be a taboo to date somebodies ex.
Communication is good and you should talk to your coworker when you are not at work about it since it won't be professional to talk about personal things at work. Your coworker might be okay with it.