I believe I knew the people. And was pregnant.
I had a coworker with an African Grey parrot, and whenever a group of us went for pizza, we brought back “pizza bones (aka crusts)” for her parrot. He loved them.
At a L.A. Raiders game, I was with my brother and his friend and friend’s wife. Wife and I were in the row in front seated among Raider fans in full regalia. We took the offered swig from their tequila bottle and are still alive to tell the tale.
I was reminded of the episode where Jerry dares Elaine to steal an egg roll from another party’s table at a Chinese restaurant. (“It’ll give them a story to tell for the rest of their lives!”)
Our CAG, Rocco, is spoiled. He won’t take pizza crust unless there’s sauce and cheese. Crusts go to the dogs, Rocco gets actual pieces of pizza. 
No and Hell no.
I think this comes closest to my own answer as well. I probably wouldn’t accept it, but not because I thought doing so was gross or dangerous or anything. I don’t go out to nice restaurants often, so when I do it is an EVENT. I usually scan the menu beforehand and consider what I might want to order. It is hard to describe exactly but, for me, these types of evenings have a set rhythm and touchstones. Order drinks, scan the menu, order appetizers, order entrees, have another drink, etc. My party will often order apps and even entrees that we plan to share among each other, but these are all things that WE chose. Injecting a rogue half eaten pizza into the equation would throw things off. I don’t know, reading what I have typed here makes me sound up tight and a touch OCD, but it’s kind of how I feel.
Would I accept it?
NFW
I’ve had someone offer the remains of a sushi roll they couldn’t finish at a sushi bar. It looked delicious, but I declined. I would eat a slice of pizza from someone that hadn’t previously been touched, but that’s pretty much my limit.
Well stated. I feel the same way.
As George Castanza said, "Eat it? For 50 bucks I’d stick my face in their soup and blow).
I have shared food one time at a bar. Yes, it was pizza. They serve it on a raised glass pedestal plate and only put it in a box for carryout. I could only eat a couple of slices and 3 guys next to me were having a good time. I offered the pizza on the pedestal and it lasted about 5 seconds.
Those of us of a certain age and inclination had no trouble sharing marijuana ciggys with strangers. I seem to remember something like that happening at Woodstock with no casualties reported.
Not from the next table, but from the next campsite. I fondly remember the time our ramen-noodle dinner was interrupted by our neighbors who offered us a big bowl of shrimp scampi. “We made way too much, would you like some?” They didn’t have to ask twice 
No, what am I, homeless? I can afford my own damn pizza.
Some of us in apartments can’t afford pizza.
In my younger days I cooked for a year in an upscale seafood restaurant. When the busboys rolled their carts into the kitchen, there was a spot where nobody (kitchen and dining room) could see them. They often scarfed down untouched food in that area.
My first reaction was “absolutely not” but then I read the full OP and completely shifted because pizza is very different.
A lifetime of pizza parties means most people are pretty good at identifying whether or not someone messed with a slice. Growing up, every party I went to with pizza just let you take a slice off of a tray like Cici’s Pizza (pizza buffet restaurant).
I’ve shared pizza at a bar with strangers at least a dozen times (possibly more). I’ve also shared apps like fries and other fried snacks.
No one was coughing and sneezing all over the food. We were also using silverware for the apps instead of our fingers.
I spent many years working in restaurants and bars, and we would occasionally graze “untouched” food (you had a pretty good idea it was untouched but not 100%) that made its way back. It was almost exclusively restricted to fries and other individual fried snacks, but some folks more adventurous than me didn’t mind a more expansive menu.
I’ve tasted drinks from people before as well but that was almost always an acquaintance of an acquaintance so not technically a full on stranger.
I don’t understand how anyone wouldn’t take the leftovers home for breakfast.
Yeah but if I ate half of my dinner quesadilla, would you take the leftovers home and have it for breakfast?
A nice sit down restaurant I’m not interested in anyone else’s leftovers. Especially pizza, though I love pizza but I never order it when dining out. I can get pizza anywhere! I’m going for the walleye or some such.
Now if I was out drinking at the bar and was asked to help finish a pizza pie, onion rings, wing dings or fries hello!
Yeah - I can’t imagine why the folk who ordered the za didn’t have the remaining half boxed up to take home. Maybe they were not going directly home and didn’t want to leave it sitting in their hot car…