That phrase is so boring! As is “Taken” or “I have a boyfriend/girlfriend”
“Having sex on a regular basis” is too crude. “With the old ball and chain” is vaguely distasteful.
I want something funny, something different and creative…so I turn to you Dopers! Any ideas?
Well, it depends on who you’re talking to. If it’s someone who sort of needs to know where they (and you) stand, the standard terms tell them exactly what they need to know. If it’s just goofing around with buddies, you can call it whatever you want.
“I’m snuggling with my bunny-wunny exclusively.”
This is one of the reasons people break down and get married*
*not me. I was smote.
For both goofing around and letting strangers know. I’d like to do something besides the standard line. Been a long time since I’ve been in a real relationship; I want to have a little fun with it!
Being in an open marriage makes this even more fun. I’m particularly fond of, “I’m somebody’s “it’s complicated” right now!” 
For quite a while I used, “This is [name], my Whatever,” but only if I knew they’d take it in good spirits.
Before I was married, at moments when “girlfriend” or “fiancée” seemed too conventional, I used “This is [name], my Person To Contact In Case Of Emergency.”
Depending on the age bracket, “hooked up” in context usually works for me, or “this is my emergency contact” if the SO is present.
Of course, I prefer “got a piece and I’m not sharing” with people I know.
“Accounted for”? “Paired off”?
(Southern) He’s my boo. (This works better if you say “Da’s mah boo”).
(Southern) He’s my home skillet.
I’ll see if I can remember any others.
(West coast, refers only to women) She’s my breezer.
This is name , you can’t have him, he’s mine.
I gottim and I ain’t lettin’ go.
The reason why I grin in the morning.
Your bedwarmer, your honey, your lover, your mutually beneficial friend.
Mind you, I always got along fine with; “That’s, you know, him.”
Off-limit?
My favorite was at my first job after college, when my (50-ish) then-boss introduced me at a party to his S.O. with the following words: “This is my lover, Taffy.”
(Also, Taffy was a dentist. I thought that was a great name/profession combo.)
This worked for us at a conference we were at: “My um, collaborator.” 
“This is the current user of my genitals.” ?
Maybe in Southern Holland. In the Southern United States, it’s “beau,” pronounced to rhyme with “toe.” And a beau is male only.
My suggestion: “Going steady.”
What? :dubious:
I first heard “boo” used by the urban poor in Atlanta. Then I heard it again later when I lived in Winder, Georgia (which is north-ish, near Athens).
Beau is for people with money. I didn’t know many of those.
Romantically enshackled.
An um-friend
As in “this is my, um, friend”
“I’m currently benefitting.”
“With whom I’m embedded.”
“Co-starring.” Or “This is my Co-Star”
“Collaborating”