Feminists are opposed to sexism towards men. The reason you hear a lot more about sexism towards women is that men are the dominant of the two genders, so sexism against us [men] holds us back a lot less. This is particularly true in tech: how many men would you guess have ever left the tech field due to sexism compared to how many women?
Also, another point: it's appropriate to use appearance to sell products, when those products are related to appearance or attractiveness. If you're marketing a product that's going to make people look and feel sexier, it's totally fine to demonstrate that by showing sexy people using that product.
Geekli.st is not a product that's meant to enhance your appearance, it's meant to enhance your career. So the women is in her underwear, why?
And in fact, in the ad, only the woman's appearance is deemed important. The man is wearing a t-shirt and shorts, while the woman is in her underwear. That contrast to me is the most obvious reason why this is sexist. The man is essentially dressed like a hacker, and the woman is dressed like eye candy.