It’s an ancient trope.
– Meat Loaf, “Paradise by the Dashboard Light”
Whereas boomers have gotten old enough that they’re remembering their dad’s and grandpa’s jokes.
I’m surprised “square up” is so mysterious to many. It means to get ready to fight; to assume a fighting stance. There’s also the meaning to make things even, like with a bill, but I hear the fighting variation more often, usually in a joking sense. “Wanna square up, bro?” means “wanna fight?”
ETA: If you just Google “square up” the fighting meaning is given first, and then the settling a debt:
What’s even more interesting is that the mystery persisted in the thread even after it was quickly explained.
I tend to give Gen X the credit for killing that particular kind of humor. I think its because we were the first generation to grow up with divorce being largely socially normalized.
Love that song. I once sang (karaoke) the male lines along with a woman who did the female lines. We both really nailed it and got applause. After, we hugged and she left and I never saw her again.
Couldn’t stand going to the local karaoke bar because that would get done several times a night. Rarely was anyone nailing it.
Wow. I’ve never seen anyone attempt it, buy I’ve only been to a few karaoke events.
If you do an image search for “Love my husband but square up” you’ll see hundreds of items bearing the slogan, as well as the “Love my Wife But” slogan mentioned in the OP.
Sheesh. It means people get exasperated. The slogans are not pointedly hostile to women or to men. Hell, I love my dogs but sometimes I want to square up. The “square up” phrase isn’t mysterious or incorrect. It’s not promoting violence, except in the eye rolling manner of “I love my kids, but sometimes I could drop them in a well.”
I’m an out of touch boomer, but I’m slapping my forehead here.
I think it’s ridiculous. Stupid. Who cares? It’s so … random.
Why announce to the world. you love your wife but get exasperated?? Like (I think it was) P.J. O’Rourke said, " It’s like “I 
My Dog” on a bumper sticker Who wants to know? Why not just stick your head out the widow and yell “I my dog” ? Yay for you.
Melanoma has a jacket you might wanna wear…
Yeah, but… it provides work for Etsy crafters, silk screen companies, entry level designers. 
Better than “I 
my dog.”
Whereas dictionary.com offers six definitions, only the 5th of which has anything to do with fighting:
I find it increasingly intriguing that there’s such a wide range of responses in this thread, from those of us who’ve never encountered this phrase used this way to those to whom it’s totally normal.
Is there a regional divide? Is this a phrase that’s used in some countries or areas and not others?
“I 
my dog” should get approval from anyone who understands the animal shelter crisis.
“I 
my dog”, on the other hand…
Reg @gnoitall just above and …
I was in high school when those bumper stickers first became the rage. I and a pal quickly got some circular stickers of the right size made which had a picture of a wood screw. So something about like this if you pretend the eraser is a flared screw head and the shaft has threads: 
Back in those halcyon days before anyone, much less everyone, carried a camera and fixed surveillance cameras were nearly non-existent, we quietly customized a lot of dog-lover bumper stickers. :evil grin:
This may be of interest:
It comes from boxing from the 1700s. I had no idea it was that old. I was going to guess it’s a generational thing, but, no. It’s apparently not. The meaning of settling debts is not attested until the 1800s.