That's ok! That is really, really ok!
Right, it's Ashes To Ashes which is an extension/sequel/spin-off/something of Life On Mars. In LoM, Sam Tyler's a copper in 2006 who gets hit by a car, goes into a coma and wakes up as a copper in 1973. He spends one series trying desperately to get home, and the second getting so much into the swing of living in the 70s that when he finally does get back home, he hates it and goes back to the 70s to be with the love-interest and GENE HUNT! THEN THEY LEFT THE SHOW AT THAT BECAUSE LESS IS MORE! In A2A, Alex Drake is a police psychologist in 2008 who gets shot in the head and wakes up in 1981 - she's been reading up on Sam Tyler's reports of the world of the 70s, so when she realises that in her coma world Gene Hunt and his cronies are all there too, she assumes her brain's assimilated Sam's fantasy and built it's own on those grounds. Again, she spends the first series desperate to get home, and the second getting into it (though she wants to get home more than Sam did, she's got a kid - cue much talking to the aforementioned kid through a television, etc).
As you can probably imagine from reading that, it's a GLORIOUS premise for a show but not one that they can drag out especially. There's really only so many times you can hear someone say, "I need to get back home!" or "Shut up you, you're just a construct of my traumatised frontal lobe" and the like. The creators/writers had sense with LoM and ended it on a high - if they do the same with A2A, I don't think anyone will be complaining. So, them shagging meaning it's the end of the show is not just not a bad thing, it's a GOOD thing - if it means they can't carry on making it well, then maybe they'll have sense and stop making it at all. This ISN'T a show built for longevity, it's not like a bowl of porridge to start the day and keep you ticking over until dinner - it's a breakfast of ice-cream, syrup and sprinkles, a mad and gorgeous burst of energy that won't last but will put you in a good mood.
So they should FUUUUUUUCK!!!