Splash…
Tom Hanks character jumps into the water and swims after the mermaid, Darryl Hannah. That romance will be over as soon as he takes a deep breath.
Or, as soon as they try to mate. (Remember that episode of Futurama? :dubious: )
They covered this in the movie, right before they jumped in the Hudson River at the very end - he’s “safe” when he’s with her, just like he was when they met as kids. “That was real? You mean that was you?!” It’s some magic mermaid thing - if he does go with her he can never return to land, so at first he doesn’t go, but then divers go in after her and he jumps in to save her.
Well, they could stay together as each other’s beards. 
That was my response at the end of the movie - “Are you shitting me? They’re supposed to be a happy couple now?!?” That relationship is not only doomed, I’d say neither of them are alive after about six months.
My vote - any rom-com where people are lying to each other regularly but somehow it all works out in the end.
I also wish to register a
Wish I’d thought of it.
“He was wrong. Tyrell had told me Rachael was special. No termination date. I didn’t know how long we had together… Who does?”
Blade Runner…real version.
Argh, I meant to put that one in my OP! What happens the first time they as a couple have to socialise with his high-power business associates?
“So, Vivian, how did you two meet?” gulp
Not to mention that Jason Alexander’s character is probably still around and wanting to parlay this secret into something.
King Kong.
I mean, c’mon. He’s a big ape.
It was beauty killed the beast.
It’s not strictly speaking a romance, but I don’t see the future as being very impactful for Thelma and Louise… Err actually…
…and a pedophile.
Brief Encounter.
The greatest British romantic movie ever.
IMHO
Richard White and Lois Lane: it’s the her creepy stalker ex watches you raise his kid scenario, only with the twist that he can kill you by looking at you or breathing on you, plus the added twist that she’s not mentioning any of that to you.