I think socializing after work is fine in a lot of cases, but I personally feel like a manager should avoid anything that even has the appearance of playing favorites. I recognize that this is probably a much more strict ethical stance than most people will hold though; in a lot of places it's not even forbidden for managers to date their direct reports, which is even more obviously a conflict of interest.

I also totally understand that there's no requirement to drink at a bar; I just don't typically find them to be very fun environments. At least in the ones I've been to post-work hours, they tend to be loud and crowded, and I've always assumed that the only reason people tolerate them as a place to socialize is that drinking makes those aspects less annoying. Imagine if all your coworkers really loved comic books, and after work they always went to hang out in a comic book store, and you had no interest in comic books; you certainly could still go, but without actually being interested in what the venue provides, it's just not going to be particularly fun to hang out there.