I think the definition of date depends on the couple. I know some couples who are perfectly fine hanging out with singles and don't make them feel awkward and like they're a 3rd wheel. I also know other couples who are all over each other (ie a makeout session at restaurant booth) and it is an awkward feeling for the single. Also every other word out of their mouths is about how awesome it is to be 1/2 of a couple (yes this is a specific example).
I think if you're not the latter couple, for the most part you're fine. But whenever there's an odd number in a group someone is going to be the odd one out. If one person in the group is consistently that odd number, that's when they're going to feel bad.
I have married friends that I met while I was still married. They've been with me through the whole divorce and everything.
We do many activities together, but they never make me feel like I was an outsider.
I have been in situations where there were whispers, and heavy PDA. I left, and the couple couldn't figure out why I left.