I don't believe that any of this is sexist at all. Most of us talking about the features that an actress that plays MJ should have have also commented about other actors and how they were not right for their role.
For example, Topher Grace was a horrible choice for Eddie Brock/Venom. Slightly because of his acting, though mainly because he did not fit the character's look. It was a bad casting choice. Eddie Brock/Venom is supposed to be built bigger, by a lot, than Spiderman and not played by some actor that would have actually been a decent choice to player Peter Parker.
Is it "sexist" to think that Topher Grace was not built enough to play Brock/Venom?
To me, Shaelene would be a decent choice if they go the Ultimate route, but also IMO the ultimate route is a poor choice for MJ. In Ultimate she's more of a girl next door than an out of Peter's league super model with an amazing personality. To many of us that version, the model who is real, is the iconic one.
I think that that is what this is really all about. The Gwen that Webb & Co. have gone with is iconic, and should be. The Spiderman that they have gone with is, for the most part, iconic, and that's a good thing.
And the same needs to happen for MJ.
Sure, they don't have to go with the iconic versions for every character, such as Electro and Rhino, and please not the iconic, over the top JJJ; but main characters that will be with us through numerous movies and will be directly connected to Peter's growth and relationships need to be the iconic version.
I think this is why so many of us didn't like Shaelene as MJ. They didn't give us iconic MJ with Kirstin, and Shaelene wasn't meeting that as well. Hopefully now they will realize what needs to be done and either glam the heck out of Shaelene, or go with another actress who meets the iconic version of MJ.