I think it could be a sign of danger or the harmless fact that you are married to a hermit. So let's discount the latter and deal with the former.
Some men distance themselves from their friends when things aren't going right in their R, because they don't feel safe, or comfortable, or whatever, talking about their R when it's not good. Some men distance themselves because they have a controlling or jealous W who complains every time they go out, and so it's just not worth it. There are many other scenarios, but I think it's healthy for him to have friends outside the R, and you should encourage it (lightly, of course), as well as integration of his friends into your (plural) life. Over time he could grow to resent his isolation (or, he could be quite happy with it...).
Does he feel like you quashed his ability to enjoy himself outside the home? Or the interactions with you and his friends were awkward because you weren't the "same kind of people"? Or he was too busy with home duties and eventually gave up on socializing with friends?
Just a few thoughts... certainly not saying this is what's going on, but it's worth a mini-investigation.
