Thanks for this. Should have said (and updated post to reflect) that I do chat and email with men online, and husband encourages this and reads these conversations.

What I worry about is that adding a third person really is different from anything else we might do together or even with a third person online. A real live man, with his own personality and wants and needs etc. adds a whole different level of seriousness to things. My husband enjoys the online chats and things but there's still a level of unreality and remove for us both. There's no real person in our room with us. No one we actually have to make room for emotionally and deal with quirks and desires, etc. It's at that level that I think things really change. And it's after that that worries me. Will husband regret it? And I think I have to rely on what I know of him and his personality. Not to be condescending to him (and I'm going to delete this thread later before he gets home so he doesn't see it) but I feel like I know him better in some ways than he knows himself. So my fear that he may regret it afterward to me feels more genuine than his assertion that nothing will change between us.