The evidence is all circumstantial at best. Much of it is actually probably speculative. I have no objection to him being gay, and I'm hopeful it gets some really interesting exploration after Secret Wars. I'm happy for the people who are excited about him being gay. I just think it's important to keep in mind that there was absolutely no real previous evidence.
Which is fine. Plenty of real people come out when no one could've predicted they were gay. There's no shortage of men married with kids who realize, "Holy ****, I'm gay." So the lack of evidence isn't a problem, though strictly from a writing stance, Bendis needed to drop hints throughout ANXM that he was leading to it. Fiction is not real life - in fiction, having things happen out of nowhere is going to make readers feel cheated. So Bendis not dropping hints throughout ANXM was bad writing.
But while the lack of evidence isn't a problem, I think it does bear noting that there was no evidence of it. And arguing that there was evidence is, to me, slightly disingenuous. It's starting with an end point and trying to justify the pre-selected conclusion. It'd be like if I decided Iceman was secretly Jewish, and then went searching for things that, taken out of context, could maybe possibly sorta prove that he's Jewish.
I'm fine with him being gay. But let's not all pretend that there was real evidence of it. And let's certainly not all pretend that his romantic troubles are because of repressed orientation when he's a young neurotic underachiever with poor self-confidence. If he suddenly starts having great, successful relationships now, then that is something I'll have a problem with. A young neurotic underachiever with poor self-confidence is going to have romantic problems no matter what his orientation.