Amazing Spider-Man 2 is picking up steam with an EW cover featuring Spidey and Jamie Foxx as Electro, and now star Andrew Garfield is asking everyone who'll listen, "Why can't he be gay?"
If you're going by the books, well, MJ is a woman. However, there's nothing saying they have to stick to the books.
There's been plenty of instances in superhero movies that the storylines don't even come close to what is/was found in the comics. So that's not really a valid argument.
Garfield goes a little bit further by saying he would like the male version of Mary Jane to be played by Michael B. Jordan.