Yeah, these things shouldn’t be an issue to anyone but a potential mate, but I think the evidence shows that some people decide to make this sort of thing their issue, for reasons of their own. Explaining this is beyond me. 
Actually, no, all of a sudden I have a theory about it, so I’ll give it a go.
I’d say that social groups are often based on an expectation of shared ground, certain traits that are ascribable to all members of the group. And that social groups can be pretty harsh on group non-members.
What are the traits of a social group? Well, that depends on the group. But it’s not uncommon that particular traits that are fairly common in the wider culture may be defining on a group – in the broader culture I’m most familiar, such traits are traits like heterosexuality, like having one’s sex match one’s genitals, like being Christian, like being monogamous. Some cultures might have expectations of ethnicity, of politics. Or other things.
Someone who makes a realisation about themselves that may be at odds with the group definition runs the serious risk of being removed from membership in the group if that information is known. (I tend to call this “being the green monkey”; there was a study at one point that removed a monkey from its troop, painted it green, and returned it with no other change, and it was violently driven off by former friends and kin.) Groups don’t often have lists of their subconscious expectations of what members will be, either, which makes it particularly rough for someone going through a realisation or a change which they expect could be taken hostilely.
And some groups are actually able to be flexible. Some, when realising that their “someone like us” parameters are out of whack with people who have been considered “like us”, discard portions of the parameters rather than discarding the person. I don’t know how common this is, overall, but it’s not universal.
Personally, I would rather a world where traits like orientation were not the sorts of things that provoked a green monkey reaction. I’d replace it with a world where being a jerk was the sort of thing that produced the same “not one of us” response, for example.
(I’d note that GD has its own examples of green monkeys: people who come in and posit their opinions without being willing to debate or discuss them, or founding them on sources that don’t pass group muster as acceptable support. The response to them looks to me like very often being “not one of us” green monkey treatment.)
