From the sounds of it, it's mostly a simple matter of the places you're looking to make friends at. Workplaces vary, but it's certainly not uncommon for associations to end the moment one walks out the door. Classes are, well, a time when they are focused on learning, rather than making friends. I hate to say it, but, there is also the issue of age. A lot of unattached mono females our age are fixated, consciously or unconsciously, on getting into that LTR they've been looking for. A guy with a gf doesn't fit that bill. The attached ones are a whole different ball of wax, with a wide margin of different issues.
So, what's a guy in your situation to do? Change the venue. A couple of options for you to consider. Go online, where there are tons of resources such as dating sites, gaming sites, etc. Go to poly meetups and events. Try a hand at poly speed dating. Heck, even non-poly speed dating might be worth a shot. In short, get yourself into more settings where being social is not just acceptable but also a major part of the experience. I'd agree that hitting the bar/club may not be the best idea, unless you happen to know of a few that aren't the typical pick-up spots.