I have no objection to sexy. I like sexy women, and I like illustrated sexy women. Frank Cho is quite possibly a minor god in my totemic pole of lady sexy.
What I object to is sexiness without taste or thought. I speak of sexy before cool.
A superhero's costume is, at it's essence, 'cool'. Even if it is hideous by our standards today, the whole aesthetic value of the costume is in its coolness. Yea, they have various levels of cool, all dependent on the character and universe and timeline, but they have to reach coolness at some level. Superman's costume, however adorably dorky, is old school cool. Batman's costume is old-school + dark/grim cool. Spider-man's is just cool.
For female characters however, sexiness tends to trump cool. Think of iconic female costumes, and you see what I mean - Wonder Woman? Basically a bathing suit. Emma Frost? A white dominatrix outfit. Black Canary? Fishnets. They all play to sex before they try to be cool. They are all about accentuating the sex appeal before the aesthetic awesomeness. This happens less with the male characters.
Not that the comics industry has totally failed. Black Cat stands out as a costume that is awesome AND sexy. I mean, it's totally in character, it's bad-ass, it has cat-burglar written all over it. And christ, it's sexy. Especially with the cleavage. It tells you that this is a woman who is slinky and lean and ready to pounce. Tim Sale's version of Catwoman is also beyond sexy/awesome. She looks like a fucking panther, a sexy death machine. Her muscles look taut and imposing, she radiates primal power and passion, and it fits her like a glove. Can we say "Meow"? Hell, they got it right with Natu. It takes the elements of the GL uniform and makes it very slick - black with the right amount of green, contrasting against her skin. And then there's the cleavage. You want to drown right in there. This doesn't mean I don't like Emma's costume. I do. But it is a guilty enjoyment. I have no problem with her being sexy - it's part of her character. But it is such a blatant approach to my sexual desires without offering the badassness that would make me feel less guilty of carrying around an issue with her on the cover.
Essentially, I think that the trend of sexualizing superheroine's costume is problematic because SEX becomes the defining feature of the costume/superheroine, and not the awesome. There is a double standard - the male superhero costume is slick/awesome/badass, the women superhero costume is sexy. They (artists/editorial/writers/industry) are defining power as sexuality for women, which is I think quite deplorable. Sexuality shouldn't be covered up and hindered, but nor should it stand as the only way women can express their power.
Sorry if this all seems somewhat confused - it sounded much better in my now-Tynenol affected brain.