Well, I originally started wearing thongs because of "fashion issue" a few years ago. I went to a real lingerie store, where the salespeople actually help you. So now I buy 2 brands of underwear; Le Mystere, which is very expensive, but I use my "birthday coupon," the store sends me once a year to replenish the supply, or Hanky Panky. Hanky Panky makes 3 styles of underwear-- v cut thongs, bikini, and regular thongs. They are lacy, one size fit all/stretchy and very comfortable. They do come up a little higher than the Le Mystere, so I can't wear them with everything, as sometimes there is vpl.
I have seen both of these brands on STP, so I think you can get them for less money on a regular basis.
Oakleaf, I think my reason for not liking the boy shorts are the same as yours. I am thin, but i have big thighs. They are not really flabby big, but big from cycling, etc. Any fat I have is my saddlebag area, even though my hip measurement is not huge. I even bought a boy shorts bathing suit bottom and it is just not comfortable and looks not so good on me. The only time I wear it is when I ride my bike to the pond, because it fully covers my rear when I ride the mile up and down the hill to the beach.