Not all gyms are the same ... the big multi-nationals tend to be the 'trendier' places and if you're going to find posers, it's more likely to be in gyms such as Fitness First etc.
Why not just go and check a few places out ... you can often get a 'feel' for a gym as soon as you walk through the door. The first time I joined a gym I was about 20st and pretended to be checking on behalf of my husband. I told myself that if I saw a single thong leotard then I'd be outta there!
As it was, the gym just had ordinary people in legging and T-shirts all doing the same thing ... trying to get a bit fitter! Nobody looks at you and sniggers - if anything, when I was even bigger than I am now, people tended to give me encouraging smiles: after all, I'd actually got off my butt and was addressing my weight problem head-on - that's nothing to be laughed at is it?
I loved my gym up North and went three times a week for two years. I moved home to the South coast 18 months ago and it's taken me this long to find a gym I like. I went for my first serious workout today and the memories of how AWFUL you feel at first came flooding back.
It takes a few weeks of dedication before you begin to feel any benefit ... until then, you can sometimes find yourself questioning why you're going at all! I know I feel shaky, a bit sicky and generally knackered at first - but then it all slots into place and you start feeling great!
Good luck - remember, look around and see what's on offer.