Folks, this one hit me last night lying in bed. Not sure if it's too obvious, but it's very short. Let me know what you think.
INTBEDROOM – NIGHT
Husband and wife lie in bed reading books.
WIFE:
Darling, when we met, you agreed we’d have an open relationship. Does that still stand?
HUSBAND:
Of course honey, as long as any partner you have has a penis smaller than mine, I’m ok with it.
WIFE:
Ah, monogamy it is then.
its seems fairly obvious to me.
I would say be more creative and unexpected
It is pretty obvious. Even as a quick sit-com exchange it's a bit weak. These sorts of jokes always make me think of a spinning bowtie.
Sorry guys, will try to do better next time.
Now I feel bad.
It sounds more like the banter between the couple in My Family, okay enough joke, but not worthy of a sketch.
sorry to butcher your sketch 
1.INT. BEDROOM. NIGHT
WOMAN is lying in bed, eyes closed, smile on face. ENTER HUSBAND.
WOMAN: Hmmm.
MAN: are you ok darling?
WOMAN: Oh yes, yes.
PAUSE.
WOMAN: do you remember when we said we'd have an open relationship, does that still stand?
MAN: as long as he has a smaller penis than me, then yes.
WOMAN: oh good, so very good.
MAN: why do you ask?
a MIDGET comes up from underneath the covers. big grin.
MIDGET: allow me to introduce myself, i'm...
MAN: whoa, how long has this been going on?
MIDGET: oh, i'd say since about 3ft 6".
PAUSE.
MAN: whats that in metric?
END.