Ugly Americans/WMG

Ugly Americans will parody Transformers.
They already have robots in this setting, only a difference of scale is needed. They would mock how often Optimus Prime dies, likely by having their Prime expy die every couple minutes. Comparisons to Kenny and Jesus Christ every time it happens too. Oh and the robot from the department of integration will fall in love with an Arcee expy. To top it all of this episode will air on the same day as the premiere for Transformers Prime.

Mark isn't as normal as he looks.
The hero is never completely ordinary in shows like this, you know... He seems to have adjusted quite well to the Destructo Nookie with Callie - he was even reading over performance evaluations while she was whipping him!

Callie and Twayne getting together won't bring forth the end times...
Callie and Mark getting married will.

Lilith is still alive.
...because why not? Hey, the only thing that actually happened to her was getting dropped into Hell - right in Callie's garden, actually.
 * Who's to say her Rapid Aging stops at 18? Assuming she has the lifespan of a normal human, they wouldn't have to deal with her for over half a year.

Mark's dad isn't human
This might be the reason we haven't seen him yet...
 * Perhaps his dad is an Angel? That would explain his last name (Lily) and also why demon women are so seemingly obsessed with corrupting him.

Mark Himself is an Angel Unaware
Like Callie, he is a Half Human Hybrid, but of a Human mother and an Angel father (who possibly had to return to heaven or risk becoming a Fallen Angel for sleeping with a human.) This would explain a lot of his Incorruptible Pure Pureness (it's implied in one epside that not cursing is even in his DNA), His innate naive desire to help everyone and everything, and the fact that demon women are somewhat obsessed with Corrupting him.

Mark is gay
He can only acheive a climax via penile insertion, and demons are known for having large barbed penises
 * ¡!¡ ALL DEMONS ¡!¡
 * Seemingly Jossed by "Mark likes Dick".

Callie's conception wasn't retconned
Frank Sinatra was apart of the cult that associated with Aldremach and Rosie was a little drunk when she agreed to that deal.

Eventually there will be an episode where they have to deal with a time travler.
And He'll obviously be a parody of The Doctor and Mark would have to deal with his remorse from being the destroyer of worlds.