This is such a hilarious prompt that I have to expand upon it. I might need to leave out the pregnant part until maybe after the wedding because the comphet that affects Shen Yuan is still a hilarious part of his character. If he knows he can get pregnant, I think that would cut down on a lot of the hilarious romantic misunderstandings.
But yeah, someone (thank you @latchkei) had the brilliant idea that Shen Yuan transmigrates into a certain subset of parasitic demons that infect their hosts' minds with nightmares, hallucinations and paranoia until their prey basically isolates themselves enough that they can feed without anyone interfering.
Well, instead of nightmares, Shen Yuan helps Binghe out by creating either the nicest dreams demonic cultivation can offer and maybe adding hallucinations of Shen Yuan to comfort him and teach him how to properly cultivate.
Fleazun is finally born after much trial and error and Binghe is completely smitten with the blood sucking parasite that loves him for some reason.
Oh my god, Binghe's "Without A Cure" moment could be when Meng Mo spills the beans about Shen Yuan's natural life span. I can easily see Binghe still beating up the one-armed demon and Sha Hualing sending Meng Mo in revenge.
This could be revealed at the end of the dream sequence where Shen Yuan gets dragged into then consequently kicked out because he's had enough blood to be a little too OP in the demonic mind arts.
Then Meng Mo can be all "I can't believe you fell in love with an actual parasite. You know those things die in like a day, right? You should get rid of it before it cripples your cultivation and mind tbh"
While Binghe can have a little OP moment of his own where he's just like "Elaborate 🤨" because Fleazun has accidentally created a complete and utter monster since Shen Yuan has 0 reason not to try and make Binghe into an immortal powerhouse as quickly as possible in the time he has left.
We can conveniently make a wild flea demon's natural lifespan around 8 years, so Shen Yuan has already invented this idea where he can die in Binghe's arms(? Hands??Hair??? How big is a flea demon?) when they finally escape the Endless Abyss together in a beautiful aged mentor arc. Shen Yuan has already picked out what kind of hallucination he wants to create in his final moments. He's going to create a tasteful backdrop of a sunset and have cherry blossoms flowing around his dying body. He's got a motivational speech planned out and everything.
Binghe then spends every waking second either attending to Fleazun and subtly using every single one-in-a-million artifact he finds to increase his cultivation because Shen Yuan is able to live slightly longer the more powerful his host is.
Also, we can extra make it a "Without A Cure" parallel because once upon a time Airplane got so drunk he created this whole subset of parasitic demons because his readers WOULD NOT SHUT UP about adding hot vampire wives, so in a drunken act of spite he added like hot flying mosquito girls, hot mermaid leech girls, tick girls, FLEA GIRLS with the caveat that they all die really quickly compared to the other mostly immortal cast. UNLESS ☝️, they feed while dual cultivating with a heavenly demon. There's this really hot sexy ritual that somehow ties the demon's lifespan with their host's.
Shen Yuan knows this, BUT OBVIOUSLY BINGHE WOULD NEVER dual cultivate with a disgusting MALE parasite like himself. Guess he'll just have to dramatically die in Binghe's arms of old age when the time comes...
By the time Shen Qingqiu chucks them into the abyss, Shen Yuan unknowingly already has the lifespan of a mortal human, and it's only going to steadily increase the more Heavenly Demon blood he consumes.
Honestly, breaking his cradle seal is probably the best thing that has ever happened to Binghe. In the Abyss, Shen Yusn finally cultivates into a much bigger, more humanoid form because of this new power available to him. He will now finally be able to help Binghe more directly. This totally doesn't awaken anything in Binghe btw.
Plus, we can make Binghe an even bigger freak when he realizes that he can constantly feel how much of himelf is inside Fleazun at all times. Maybe get him weirdly into the parasitic relationship, because that means Fleazun LITERALLY can't live without him because the only blood Fleazun wants is Binghe's (for completely heterosexual reasons. Definitely. For sure.) and his blood is the only thing that increases Fleazun's lifespan with 0 harmful side effects (besides the whole "Binghe can control Fleazun's entire body completely with just his thoughts" thing, which Binghe doesn't have deeply sexual thoughts about)
Then, it finally comes out that Binghe can tie his lifespan with Fleazun through dual cultivation. Maybe through unbelievable shenanigans Shang Qinghua ends up spilling the beans on that.
By then, it is then so over for Shen Yuan. He will be Binghe's pregnant flea wife before anyone can even blink.