Guilt is a sign that something’s awry in your moral world. Your guilt is signaling you that you’re doing something that you don’t think is right. I can help you understand your feelings. 🙂
Since you’re not married, you’re free, and even obligated to make sure you’re with Mr. Right, if monogamy and/or marriage are your goals. So if you think this old friend is a possible Mr. Right for you and your current boyfriend isn’t, then it’s fair for you to date him. But make no mistake — that’s what you’re doing. You may not be kissing or having sex with this other guy, but he’s clearly interested in you, and knowing you’re not married, he’s making a play for you.
Your current boyfriend is probably of the mindset that if you belong to him, this other guy won’t be a threat, and that’s why he’s pretty calm (to you, at least) when you tell him you’re seeing this other guy. So, consider what you want in your life, and where your current boyfriend fits in. If you still want to explore, and if you’re fine knowing that at some point your boyfriend may turn the tables and want to see an old ex-girlfriend of his, then proceed with caution.
However, I think you’re going to find that this old ex-boyfriend will put the moves on you if he senses mutual interest, and that you will eventually have to make a decision that will allow you only one of these men in your life.
I hope that helps.