Gender is optional. How do they reproduce? One thread I read said they were human with the blood of dopplegangers like tieflings have a spot of demon blood, so do they choose a gender and procreate in human fashion or are they perhaps asexual? The listing in ebberon referenced families so asexual seems unlikely, does one choose to be female and stay that way for 9 months? Can they mix with other races with an equal chance the kid is all mom or all dad? Can I get a half changeling/ half dragonborn character race option that can shapechange into any color dragonborn? Half change/gnome who loves to tinker but sometimes looks like a dwarf… half tabaxi but can mimic any thundercat? 

This is up to each individual GM, so ask your GM how it works at their table.

From Keith Baker's blog:
Can all changelings get pregnant? Are they biologically asexual and just choose their current sex with shapeshifting? Yes. What’s been stated in the past is that changelings set their sex with shapeshifting. Prior canon has said that a pregnant changeling actually loses the ability to shapeshift during the pregnancy. This seems extreme to me, but I could see the idea that they need to maintain a female form in order to maintain the pregnancy (and that shifting form very early in the pregnancy would simply end it, so changelings have a very easy form of birth control). The idea that changing sex is an instinctual thing, like flipping a light switch, and that a normal changeling couldn’t, for example, assume a male form but keep the uterus. With that said, if you had a changeling called out as having greater control over their abilities (for example, the Changeling Menagerie druid I’ve mentioned elsewhere) I might allow that.

I recall from the 3.5 edition Eberron sourcebooks that Changelings have a specific gender they default to (and they can change their biology accordingly), but it doesn't manifest until adolescence. Even so, I expect a lot of them are fairly fluid in that regard post adolescence nonetheless.

And changelings can procreate with any humanoid race, with a 50/50 chance of the offspring being of the race of either parent (I believe it's not specified how that works with a parent who's already from mixed descent, but let's not overcomplicate it and assume it's the same).

That said, all of the above goes for Changelings in Eberron. In any other setting go with XXXGammaRay's answer: ask your DM.

More importantly as my table spent 45 mins discussing last game, do you refer to a changling as their gender right now, their natural gender, it, them, they, are the binary, non binary, considered the same as a trans or are they cis. If a male changling is presented as a female and is seducing a male target what do you class their sexuality. 

Such are the questions that my players came up with when an NPC simply mentioned as an aside that changlings exist in my world. The characters are now hunting one down to ask all those questions of. 

Thanks guys, great feedback.

Scarloc… your table has some morbid curiosity issues that may require one of them to devote a proficiency to therapist. Kinda jealous. My table has a gnome that never asks questions and believes he can camouflage by putting on a garden gnome cap.

Quote from Scarloc_Stormcall >>

More importantly as my table spent 45 mins discussing last game, do you refer to a changling as their gender right now, their natural gender, it, them, they, are the binary, non binary, considered the same as a trans or are they cis. If a male changling is presented as a female and is seducing a male target what do you class their sexuality. 

Such are the questions that my players came up with when an NPC simply mentioned as an aside that changlings exist in my world. The characters are now hunting one down to ask all those questions of. 

 I think all those questions should be changeling specific. I don' see how any of those things are the same for each indiidual changeling. I mean even we humans with our relatively unchanging bodies will answer all of those questions on an individual basis.

My changeling goes with pronouns as per their gender right now (we play in german and german doesn't have a solution like "they" that doesn't sound derogitory), but changes gender however they want. As per sexuality, that to changes per form right now because they have a bit of a problem regarding finding their true identity (right now they walk around as a copy of their best friend growing up)

Quote from Scarloc_Stormcall >>

More importantly as my table spent 45 mins discussing last game, do you refer to a changling as their gender right now, their natural gender, it, them, they, are the binary, non binary, considered the same as a trans or are they cis. If a male changling is presented as a female and is seducing a male target what do you class their sexuality. 

Such are the questions that my players came up with when an NPC simply mentioned as an aside that changlings exist in my world. The characters are now hunting one down to ask all those questions of. 

If they're having fun and everything's done in a respectful manner, more power to them. It's an opportunity to do some worldbuilding at least. Most of these seem like they'd be answered by applying common sense and some basic courtesy though. Their sexuality is what it is, not necessarily what their actions suggest - seduction doesn't have to be about sexual attraction, after all. They're neither cis nor trans (since those terms mean you were born with your gender matching your sex or not respectively, which doesn't apply to changelings); I expect the closest equivalents we have in English is something like androgyne or intersex, although sequential hermaphroditism is technically more correct (but arguably a horrible term to apply to a humanoid). How you refer to them is ideally how they want to be referred to; that's the same for everyone, changeling or not.

Well spoken Pangurjan

This exact topic is now an inside joke with my table. Last campaign we had a changling and during one long rest, one of the party cast sending at midnight to the changling saying "If you were pregnant, and shapeshifted into someone without the anatomy for pregnancy.... what would happen to the baby?" and we explained it as the changling saying "iunno" and just rolled with it as a perpetual mystery lol

Quote from Scarloc_Stormcall >>

More importantly as my table spent 45 mins discussing last game, do you refer to a changling as their gender right now, their natural gender, it, them, they, are the binary, non binary, considered the same as a trans or are they cis. If a male changling is presented as a female and is seducing a male target what do you class their sexuality. 

Such are the questions that my players came up with when an NPC simply mentioned as an aside that changlings exist in my world. The characters are now hunting one down to ask all those questions of. 

It's simple, they're fluid.  A conversation insisting on heteronormative centered language could and is trying to pin down a changeling by assigning it a sex or gender or some other linguistic conditioning, but the effort fails.  To play off the actual race/lore available in 5e's Eberron, such a conversation actually assigns a mask to what a changeling "is" in the moment and literally neglects the totality of what the changeling is as a being, instead choosing to cover it up in assigning it a "necessary" term.  Insisting on needing a name or a pronoun for a Changeling in a specific circumstance is trying to control a being whose existence challenges conventional notions of identity on more grounds than just sex and gender.  This isn't hard, it's imagination ;)

Like everyone go read/watch Orlando or something or some more contemporary stuff in its wake like Winterson or Butler.