It's not odd at all. I've been on three family trips as a solo, and it was just fine. There were also families with adult kids, moms with adult sons, moms with adult daughters, couples with no kids, etc. So I wouldn't worry about that.
Personally, I like the dynamics of an Adults Only trip, because it seems to me the Adults can feel free to be more child-like. I suppose it's a personal preference. I don't dislike kids or anything like that, but I just prefer not traveling with them.
The itineraries are, in general, the same, although sometimes, depending on the trip and the circumstances, the Guides might include a more Adult version of an activity on an Adults Only trip than on the family trips. In Alaska, we did a brewery tour that included lunch rather than a more family-oriented restaurant. On my Greece trip, the Adults got to do the race in the Olympic Stadium since there were no kids to do it. Stuff like that.
Sayhello