This is a good example of why women are getting slowly but surely more stupid as time goes on--their brain-power is being eaten alive by conundrums like this.
Actually, I just feel that an alternate viewpoint should be presented, and of course by that I mean mine.
Try this as an alternative: Every time you worry about your underwear, use it as a good opportunity to laugh hysterically and move on to something more interesting. Do not, under any circumstances, buy underwear that makes it look like you're not wearing underwear. You have just spent ten bucks or more to change nothing.
Or maybe I'm just lazy. Here's my female underwear table:
Briefs or Bikinis: If I bought them in the first place, it's because they were cool and didn't bug me. These are for general wearing. These are underwear; these are what I mean when I say I'm wearing underwear. Er...is that too complicated? Oh yeah, there's more....
Thongs: Yeah fucking right.
And of course, when I'm put in a situation where people might laugh if they knew that I was wearing underwear (though I don't know why; most of us wear it...), I just go without. Funnily enough, this is considered less socially acceptable than having your panty-lines showing, but for some reason, when gossipy little old crabby ladies see a lack of panty-lines, they just assume you paid for them. I get my lack of panty-lines for free. Whee!