I understand your reluctance to wear a Speedo ... I wear
nylon running shorts with a lining. There is very little drag in the swim, and they are fine for the bike and run as well ... they dry quickly and don't require a change in the transition area.
I've seen a lot of guys wear bike shorts the whole way, but to me they don't look very comfortable.
I'm overweight also I use some spandex tight black underwear. Yeah their still skin tight but at least there not showing half my butt cheek. Also I assume when you wrote sprint that you meant a sprint triathlon. If so then when you are in the transition area you can throw on a pair of shorts over the spandex tight underwear. You know I'm self conscience of my weight also but look at it this way how many people are thinner than you are that don't do jack about there physical condition. Your out there and should be proud of it. After that triathlon you need to wear those painted on numbers on your arms and legs like a badge of courage, in fact go over them again with marker before you go back to work so you can say in your face couch potatoes!
When I do triathlons, I often do the whole thing in just a jammer. It's basically a knee length swim suit. You can find these suits at my website, www.swimyourbest.com. Enter the coupon code, "openwater" for 20% off at checkout.
I recommend a tri suit as they are made now for all 3 events and you don't have to change during transitions. I purchased a 2 piece Louis Garneau Shark tri suit and the material is designed to let water flow over the suit, has a small chamois like bike shorts do for the bike, and wick away moisture quite well during T1 and on the run. The bottoms look like swim jammers, so they'll help with adding some support to your thighs. Many tri outfits are also made to be worn under a wetsuit and won't fall apart like a normal swim suit might. Any online tri shop should have a good selection of these. I purchased mine via www.all3sports.com. Good luck on your tri!