I've chimed off on this before (re: FC sexuality), but I don't think the concern is so much on the sexuality of the character. I think it has more to do with how the character will behave under the control of the player.
I have no issues with a female Captain America or even a gay Captain America (there might be a compelling story to tell in Steve Rogers arriving in the modern age and realizing that he can be gay in 2017 with a certain amount of respect that wasn't prevalent in 1941).
I like to think that players would approach those gender/sexuality issues as a means of compelling story.
HOW and EVER...
I think the real concern people have is in a Captain America transported to 2017 as FUCK EVERYTHING THAT MOVES BONDAGE SEX SWING NEVER INVOLVES IN ANY STORY AND SENDS EVERYONE DICK PICS Captain America, thus really putting aside any meaningful role playing with the character concept in favor of using the character as a sandbox Shang Alt.
We'd seen mentions recently of gay fish Daken uncomfortably hitting on people IC and OOC, and mentions of Sex Magic Sorcerer Supreme Dr Strange.
So, in summary, I think the concern is less on gender/sexuality of the FC and more on whether or not they'll be used like a Harvey Weinstein extension of the player's whims, thus becoming more of a distraction than a contribution.