We always try to sit like this at a square table:
x l_l
x
So we are at each corner. That way we can be close, and still be opposite each other to talk.
But in a booth, or anything else, we always sit opposite each other. We’re not old, and not even married. But we do like our space!
Ugh this didn’t come out right. If we’re at a square table, we sit on two sides close to each other, rather than across. Perpendicular, not parallel.
Yeah, but it’s etiquette. Like the guy says.
Old (or at least long-time) married couple here.
We usually sit across from each other in booths (occasionally side by side, but it’s very rare.) If we’re with another couple we’ll sit side by side. At tables (which we avoid if there’s a booth available) we sit diagonally across so we can talk more easily.
One thing about me, though: I am very reluctant to sit in the inner part of the booth, and prefer sitting on the outside, especially if my seatmate is male. I think it’s a combination of resentment that my parents used to stick me in that spot when we ate out (presumably to keep me from getting up and running around, though I wasn’t prone to doing that) and some kind of tomboy “I don’t want the guy hemming me in” thing. It’s the same reason I won’t sit in the middle if there are three people in the front seat of a vehicle and the other two are guys. I insist on the window. (This doesn’t come up much nowadays since cars no longer have bench seats, but sometimes it does in pickup trucks.)
BTW, I’m not obnoxious about this–I usually make a joke about it, and if the other person is as adamant as I am I’ll give in about half the time. I just don’t like the assumption that the woman will sit in the inner part of the booth or the middle of the vehicle seat.
Like most the OP, we sit across from each other at a booth and next to reach other at a table. For us the major consideration is putting me where I cannot see the TV (if the bar/restaurant has one). I have no immunity to the glowing box and am unable to make conversation while so distracted.
Ah yes, the accursed TVs. They distract me, too, and I don’t like TV all that much.
Like the OP, across from each other when alone and next to each other in a group. Not old marrieds, either - 21/22, living in sin, having recently celebrated our 4th anniversary.
There is a sense of hierarchy when we sit at a table…
I sit at the table where I can see as most of the room as possible, usually with a wall to my back; I don’t like to sit in the middle of a room and will request to sit elsewhere if brought to the middle.
As I sit down, I instruct my 16 and 8 year olds to sit on either side of me so they won’t tease each other.
My wife and 3 year old will sit in any remaining chairs, but usually my wife will sit between my 8 year old and 3 year old for the same reason stated above.
This set up leaves us with the least amount of commotion during our dinner. I’m still trying to think how my wife and I used to sit before we had kids…let alone when.
Sitting side-by-side reminds me too much of pigs at a trough, so I don’t do that if I have a choice.
The only time Ardred and I sit next to each other in a booth when alone is if we’re playing a tv trivia game at a sports bar and both need to see the TV.
Otherwise, it’s across.
Plus, we’re… um… plus sized people, and need our space.
We sit across from each other at a small table, or on the south and east sides at a larger table.
I always get to choose which side to sit on, because Ardred thinks it’s chivalrous. 
We sit across from each other at a booth or at “corners” to each other at a table.
I get to choose the table and the seat I take because I have this thing about not wanting to look at the kitchen or the bathroom, not even from a distance. Don’t even want the doors in my line of vision.
My husband usually watches me sit down, looks around before he sits down and says “you gonna be ok there?”
Any time I’m out with 1 person and it’s a table, I seat them on my left so I can hear (bad right ear, like others here). If it’s a booth and there’s only 2 of us, I don’t want them sitting next to me, except for my husband.
When I’m with my husband (12 years) we sit together on one side when we’re with other people (I don’t want to talk to him in that case, I can do that anytime!). Sometimes when we’re alone together, we’ll sit on the same side of a booth. I suppose it might have to do with the size of the table and/or the atmosphere of the place.
I like being on the inside of a booth seat. It makes me feel cozy.