It doesn't necessarily have to mean that women's sports aren't worth watching. Sexy uniforms would make them more appealing whether they were worth watching already or not. And if people found women's sports not worth watching, would that necessarily be sexist? What if men are generally better at sports than women? Or what if people prefer to see big, burly bodies playing sports? Wouldn't that be completely fair? Or what if more men like sports than women, and they'd just rather watch other men play them because they relate to them more / they're in-group? And I don't think the other side of it, preferring to see women in sexy clothing, is sexist either. It's just the way men are built. We love women, and sexually we're very visually oriented.

Also the people who choose to put them in sexy clothes are probably corporations, and the decision would probably have been made for the sole purpose of making more money, though of course that alone doesn't make it impossible that sexism would be involved--it would just be solely on the part of the viewer whose biases trickle up to corporate decisions.