Why? The creator never intended him to have a sidekick named Cho; if he had intended to have a sidekick, he'd have been given one. He wasn't; that implies he wasn't meant to have one (unlike Cap). I see no difference, both are things the original creator never intended, that doesn't make it a retcon.
Firstly, X-treme X-men Herc is an alternate reality Herc; we're talking about 616 Herc (who is the one who slept with Northstar). Secondly, I think it's easier to understand this when you have a good knowledge of what bisexuality means. Someone can be bisexual, and yet live their whole life never even kissing someone of the same-sex in a sexual way. Sexuality is not based on "body count"
You can be bisexual and still spend your life in primarily str8 relationships (look at Anna Paquin; she's married to a man, but she identifies as bisexual, despite the likelihood of her never being with a woman again). Or you can be bisexual and primarily only be in same-sex relationships. Bisexuality is someone's sexuality, not a score chart of how many men and how many women they've been with. Nothing about Herc's bisexuality erases, contradicts or discredits his previous relationships or affections. Therefore it is not a retcon.
Another way of looking at it. Herc wears green and gold, one could assume it's his staple because he likes the colours; but if the comics revealed his favourite colour is blue, does the fact he's wore green and gold make it a retcon, or merely a new revelation/information? There is a difference between
new information and
contradictory information.
Posters throw around the word "retcon" far too freely, without actually realising what it means. Ultimately anything a fan doesn't like they label a retcon, when 95% of the time it's nothing of the sort. They just don't like it, and therefore don't want to accept it.