The Spark Is Gone
There's been much debate as to the actual existence of the Moonlighting Curse. The demise of the hit show came about quickly after the leads, played by Bruce Willis and Cybill Shepherd, had a steamy hook-up. Conventional wisdom dictates that you can never get rid of that UST (Unresolved Sexual Tension) and keep your show afloat. After series as varied as Wonder Years, Cheers, and Battlestar Galactica survived character hook-ups, it was time to rethink the curse.
What kills a show is not the sex itself, it's death of the spark, the end of the explosive chemistry that drove the show. It's hard to argue that The Office got better after Jim and Pam tied the knot. No one's most treasured memories of the X-Files came after Mulder and Scully got together. Characters can hook-up, it just can't be at the expense of their chemistry.