I think it is more than just that though. I am sure the whole shipping thing is all it is to some people but to a larger portion of the fanbase I think it is something different. In lots of shows (and I don't just mean superhero shows) a lot of the female characters are hated. What these female characters seem to have in common is that they do not simply live to serve and support the male characters. Most of them are strong, independent women who do not rely on male characters and go after what they want and do what they want to do (this describes both Laurel and Iris very well IMO). Difference is that male characters are loved for doing these thing but females are hated and it is normally by male fans.
Skylar and Marie on Breaking Bad and Megan and Betty on Mad Men were hated too. They were characters that did not live to serve their husbands or the male protagonist and in certain cases went against them (and they were in the right to go against Walt and Don, who are both clearly in the wrong the vast majority of the time). I did not watch Soprano's or Boardwalk Empire, but Carmela Soprano and Margaret Schroeder were also hated by many people and I wouldn't be surprised if it were for similar reasons that I mentioned earlier in regards to the other female characters. There are lots of other hated female characters too. It is one thing to not like one or two female characters because they are bad or poorly written (of course there will be some female characters that are badly written, badly acted or are just not nice people that there are valid reasons to dislike) but if someone has a problem with lots of them (there are people who strongly dislike every female character I have mentioned) then the chances are that the problem lies with that person. There is no way all of these female characters are bad or poorly written. People will hate a female character much more quickly and easily than they will a male character. Male characters are cut much more slack and a lot of them are loved for doing some of the things that certain female characters are hated for.
A lot of female characters are hated unfairly and for silly reasons that men would not be hated for and Laurel and Iris are examples of this IMO (although it seems to me that Laurel is not really hated anywhere near as much as she used to be).