Bleeding Cool have received some well sourced information that may explain, in part, certain decisions made regarding Marvel characters and sexuality of late. The rules appear, however, to have been applied a little inconsistently.
We mentioned how a same-sex relationship story was originally planned for the Secret Wars: Secret Love romantic anthology oneshot, featuring alternate universe versions of Kate Bishop and Ms America Chavez. No entirely satisfactory explanation was given.
However I am given to understand that in the House Of M series, it was originally planned that the alternative universe versions of Quicksilver and Namor, plotting against King Magnus, would be in a same-sex relationship together.
The reason given why this was shot down by Marvel was that they didn't want to suggest any character was gay unless there was a lasting affect.
Which does have some validity, but doesn't stand up that much in a series set in parallel universes, and where other characters with alternate sexualities to their Marvel universe versions have been established without necessarily lasting effects, such as in the series Runaways with Jubilee and Pixie.
Although inconsistent, it may explain the Secret Love decision.