Most in the world seem to like very much operating under the assumption that everyone is straight until they come out. This is the current machination of society and just because it's not changing anytime soon doesn't mean that I can't be part of the eventual change. A brave new world where I'm not socially obligated to make some grandiose speech on something as basic as having brown eyes.
That's the way it should be. Why should the gender of the person you're in a relationship with matter one bit? It should just be a piece of data, not a life-defining label that people feel compelled to attach a morality to. No one should ever need to come out; it's unfortunate that at the moment the world is so rigidly anchored that it actually "means something" to come out.
I don't know that the parallel between this and the polyamorous situation holds up. Dating others in a closed marriage is a big no no. But where is the moral problem with same-sex dating while assumed to be straight? The whole poly thing gets easily confusing, I know. Where is the confusion in seeing two men kiss? Or two women? I think that speaks loudly and clearly for itself.
In the USA, we've barely had gay marriage anywhere until the last few years, it was 20 years ago that many states specifically prohibited gay marriage in order to publicly make a referendum on its immorality (according to the lawmakers), and before THAT, well, if you were in a same-sex relationship you were at best misled, confused, or screwed up by your parents and at worst were a communist, a pedophile, a lunatic, anti-Christian/God, and/or so forth. You couldn't be openly gay without losing your job and being driven out of town in some circumstances, if not beaten. Oscar Wilde died in prison a century ago (as an example) because he was gay; other famous people who happened to identify as gay or lesbian have died as well due to entrenched heterosexual norms.
Anyway, I guess my point is... give it some more time. We're barely past the boundary right now. The current generation doesn't seem to care much at all for the old rules, and once they begin to dominate the culture, well, that issue is settled.
.... to tie it to type, which was the topic... what would you expect an NTP to say? As far as morality goes, I don't care what gender another person mates with, I evaluate it by the bond/relationship if anything. That's actually how I always felt, even when raised within a stricter conservative Christian environment; I was told the Bible said one thing, but for me personally it didn't seem to make any sense.