It makes sense from a why standpoint. I can’t help but wonder about the how.
How did we evolve reproductive organs that are not only sensitive, but also release reproductive cells at climax?
I’m ok with a “we have no idea” answer, but if there is an answer I also want to know.
So your first paragraph is a bit irksome.
Because it’s speculation to preserve the three explanations for all evolved traits that you know about.
(1) easier to survive -> killed during the mating or birthing process
(2) easier/more survivable reproduction -> otherwise the females don’t mate (3) makes the mate more appealing so others want to mate with you.
Now hold the duck tract up against each one: Not (1). A corkscrew oviduct isn’t a survival trait, it’s purely reproductive plumbing. Not (3). It’s not making anyone more appealing. It’s not a peacock tail. Nobody’s choosing a mate because of a more convoluted tract. Ipf anything it’s invisible to mate choice entirely, it only matters after copulation. Not (2), and actually the opposite. The duck tract makes reproduction harder. It’s a filter to block fertilization from certain partners.
And finally, we have enough evidence to show that this is a heritible trait. So what’s going on?
This is called antagonistic coevolution. This is an evolved trait that makes it harder for all partners. The tract can relax for preferred partners making reproduction easier. But not as easy as a straight tract.
So we have convoluted tracts for the females… What do the males do? They create corkscrew penises. This goes back and forth, male adaptions force female adaptions. There is no directional selection of a fixed trait. There’s coevolution based on frequency dependent selection. It’s the Red Queen dynamics (that’s the “it takes all the running you can do to keep in the same place” line from Alice in Wonderland). It’s where male and female traits keep escalating against each other rather than converging on one fixed “best” trait. Rather you get a lot of opportunity for variability across the popultion and through generations. This is causal without being teleological.
Finally, there seems to be evidence that this can end in mutualism, but not guaranteed. I’m a little surprised you weren’t more curious about this little factoid when I presented it and chose to close on your best understanding.
Anyway. I believe in unintelligent design.