Search Results: go for an ex's friend

So my ex and I have been separated for almost four years now. We're not in contact anymore and that's okay. Our relationship didn't exactly end well but it could have been worse and after a while we sat down and talked about everything and parted our ways on good terms.

Now I have met one of his friends (I'll just call him X) by coincidence. I still know him from that time although we lost contact about 5 years ago when he left school and started working. We were pretty close platonic friends and I always found him attractive. My ex was a little jealous although there was no reason as I never met X without him and never had any feelings for him.
The two of them are still friends now, they're not as close as they were once but they still meet up every few weeks.

So after I met X by coincidence, we decided that we'd go out on a drink and catch up on the last few years. It was an awesome night and we really hit it off. He ended sleeping at my place and although we didn't have sex that evening we ended up having it later on multiple times.

Now things are starting to get serious. And I really like X, he's an attractive, amazing intelligent and funny guy but it's kinda strange that he's still friends with my ex. I don't know why it even bothers me. The relationship ended four years ago, that's enough time to get over it but I still feel a little guilt.

What would you guys do at my place?