what Richiev means is that in a logline
the MANDATORY information is:
1. the story?s first major event or ?Inciting Incident?
2. the hero?s function or role in the story (e.g. a mother, a cop, a scientist)
3. the hero?s goal or main action in the story

while the OPTIONAL information (unnecessary if implied in the Inciting Incident) is:
4. the hero?s weakness/flaw (e.g. headstrong, timid, solitary, cocky, depressed)
5. the obstacle(s) and/or the Antagonist
6. the stakes

yours isn’t a logline. It’s an event.

don’t lose hope, not yet..

But how do you make them (the audience) want to watch that event.
let’s do all the necessary steps to make them want to watch event “Man who used to be a woman gives birth to a baby by woman who used to be a man”

how human psychology works is you have to persuade them into liking something (I know it sounds dark but that’s how we are, so deal with it)

Aristotle stated 3 principles, Ethos, Pathos & Logos to convince someone into liking something

we’re going to do the same

the last cause in a chain of causality should be emotional because when all seems to fail he realises that he just another human….and it’s the warmest way to wrap things up

The trick lies in reverse engineering your “perceiving linear plot” into asking yourself how to stage this last cause (of causation) to serve the Pathos
and the rest as a faithful buildup-logics (the Logos) which usually sacrifice itself for the sake of emotion

Still, the health of the Logos affects the strike of Pathos all compiled into one neat system by the in-credible Ethos

in applying Pathos, keeping an end in mind forces us into thinking a cause which makes the effect (your event) affect us emotionally

let it be: they decide to create baby after such “trying” circumstances

so now our sole job remains to create “trying circumstances” for the event “Man who used to be a woman gives birth to a baby by woman who used to be a man”

what could these “trying circumstances” be, which would force us into wanting to see, this event you stated?

Time to apply some Logos..
let’s take for instance character A (a man) & B (a woman)
both gay people
(becomes a hook after the catalyst beat, of the plot I am about to suggest)
but with an exception…..they feel an unusually strong urge towards each other but never tell one another
(because-let’s be frank-how much could it help when A knows B likes only women & B knows A likes only men)
this I’d call as the “preconceived self”

so they decide-both without making the other know-having a sex change operation & hope to jump into true love..

but-you guessed it-both changed their sex around the same time

So what is the first emotional response you have for such a situation
Pity (Pathos) right?

(after which, they finish explaining everything and decide to marry each other…spoiler! kill me)

but notice how the buildup in a good plot gives the necessary structure for the reveal (Pathos) to stand in itself
.
.
that’s Logos
PS. Ethos is just production value 😀

Anyways, That could be the plot with your event as the final reveal of the total system

In which
Inciting Incident could be “a gay guy falls in love with a lesbian girl”
his function could be “confused yet curious” (and a little intrigued)
His goal “to be near her as much as possible”
His weakness “his preconceived self identity”
His obstacle “her preconceived self identity”
The stakes “a loss at chance of finding true love”

Then the logline finally becomes:
“An intrigued gay man falls in love with a girl only to find out she is a lesbian”

you don’t have to tell the complete story (especially save the reveal for finale)
do not tell: how they both are curiously confused to fall for each other while keeping the truth to themselves up to the point of their sex change operations so the other individual can love them fully, truly…

with a metaphor: true love is to change your “self” completely for the one you love unconditionally…
.
.
only to find out they are back to square one

but they realize the motives and decide instead to marry one another

so your event “Man who used to be a woman gives birth to a baby by woman who used to be a man” becomes something I’d want to watch-only after-such trying circumstances