Wow, I am amazed that this thread has gotten so much attention and sparked such an interesting conversation. I would love to see some guys here try bikini bottoms, too. I have never tried them on in stores, always purchased them then tried them on. Once you try a few on and see what fits, you will be able to tell if they will fit just by looking at them. The width of the crotch is most important, ones from Aerie are way to narrow, but the Victoria's Secret and Roxy ones fit great.
Infact, while perusing ebay I found the same ones I have, for sale. I am not selling it, not affiliated in any way, just thought I would share it if anyone wants to try a style that would definitely fit a guy.
http://www.ebay.com/itm/Roxy-Bikini-...em257007 9d53