Honestly, you peeps have got nothing to support your claim. There's no way to prove that the guy is not having a fling with another man, and no way to prove that he's not faithful. The evidence for both is zero. All you've got is chatter and speculation.
To be frank the best argument I've read is the one from dqvdqv : « .... think about what type of manga this is. » Oh yes, if you look at it that way, you gotta admit that one of Ed or Ian being found to be a cheater would be a quake to the story, alter the lives of a lot of mcs and bring some big motherfecking changes... haha. Really, this is not that type of manga.