Some women like men in sexy attire (like jeans, shorts, and speedos) so if you've got a good body, they'll probably like it.

Another thing to consider (at least according to the so called "sexologists") is that most women are not easily aroused by purportedly sexy attire so they might react to guys wearing a speedo in a purely utilitarian manner rather than seeing it as something erotic.

because.....
 

....they are looking for pert butts - and the tight-fitting speedo will accordingly impress :)

I DVRed the US Swimming Nationals I will have to but a shirt like that soon.