Fun Coop games to play casually with my SO?

Castle Crashers,
Battleblock theatre
Diablo 3 (I know, i know, not on steam)
Terraria
Marooners?

I played Castle Crashers with my SO, it was so fun and i really recommend it.

Also check Overcooked 2, Portal 2, and for sure Borderlands 2

Blackwake, soon you are gonna yell in pirate speech when you two have sex.

I mostly play Minecraft with my girlfriend, setting up a server is super easy.

resident evil 6 have co-op
trine 1 and 2
lara croft and the guardian of the light
lara croft and the temple of osiris
(i think) metal slug games
(i think) castle crashers

What means SO???

If you're talking online games, than MMOs are classic for this. Create characters and adventure together. Most MMOs are free to play, so you could try a few to see which ones you like.

I read all the comments and picked killing floor 2 today, maybe i will buy one more game
Going to re read the comments again

im still a big fan of Left for dead and killing floor

disclaimer:
i dont know whats meant with "SO"

genital jousting

Don't Starve Together
Dungeon Defenders
Orcs Must Die! 2
Portal 2
Golf With Your Friends
Escapists 2
Borderlands 2
Worms Revolution
Slap City
100% Orange Juice

Enjoy Killing Floor 2, it's a lot of fun coop! The original KF is also fun, same with l4d2/vermintide 2 if you're both into those kinda games.

what is a SO?

100% OJ now with co-op mode= )

Dominions series :'D

Serious Sam HD: The First Encounter
Serious Sam HD: The Second Encounter
Serious Sam 2
Serious Sam 3
(I'm sure upcoming) Serious Sam 4
Nine Parchments (make sure you have 3 other people for this (counting your wife) or even on the easiest difficulty level it can get frustrating.)
Painkiller: Hell and Damnation. Like a Halloweeny Serious Sam.

I have to second already mentioned games such as:
Portal 2
Lara Croft and the Temple of Osiris
Lara Croft and the Guardian of Light
Lovers in a Dangerous Spacetime (local only but if you have an extra friend or 2 who likes to get into it also, it's even better)
The Secret World

Since this game is never mentioned - Kalimba.

Is is one of the best puzzle platformer I have ever played, had a blast with my friend. It has more singleplayer levels, but there are co-op ones as well which require really good coordination. If you decide to purchase it buying DLC bundle is a must, you get 20 new levels, 10 sp and 10 co-op. Wait for Steam sale, I got whole package for 4€.

Overcooked.

Hammerwatch, Magicka 1-2, Helldivers are great games that actively promote cooperation (and Hammerwatch has insane difficulty personalisation - hp, number of lives, damage , you can play at difficulty level 1 or 11)
Saints Row 4 (and the others) sounds like a super fun game to mess around in with a friend or in your case your partner. Just beware that missions aren't always (ever?) instanced in 4, so one of you can progress the story without the other one seeing/experiencing anything from it, it's not a fun experience.
We Were Here (free) or We Were Here Too are absolutely fantastic if you enjoy puzzle-solving together - they heavily rely on communication. Each game worth playing twice, to experience both sides of the gameplay.
Based on my experience with a friend - Borderlands 2 (and up), Portal 2, Factorio if that is your jam. I had heaps of fun playing BL2 with a similarly crazy friend, and playing Factorio with him too - turns out one can really enjoy BL2's whackyness while being good at basebuilding and planning forward :D
FEAR 3 can be fun if quasi-horror games are your jam. Has ridiculous, over the top amounts of blood (in environment) for shock value... otherwise 3rd is the least horror, though cheap jumpscares.
Lara Croft and The Guardian of Light is a powerhouse of a co-op game. It is relatively simple, has great action, smooth as butter (runs on potato too) and many parts actually have puzzles where you two have to think together and work together, while not being obnoxiously hard.
FORCED also has a really cool co-op mode you could try if you own it, I think you can get pretty far in it by playing casually. Doing the optional objectives/challenges is like hell in that game, requires such precision and planning, both positioning and gear. Try how it works out for you if you own it, but I do not recommend you getting it because I feel it is risky

You got a lot of good ones named by the others.
Outland and Saints Row 3 are two games I played in coop and enjoyed them. Maybe they deserve a look.

Viscera Cleanup Detail is a really fun game

Have a look at this group, dedicated to the exact thing you are looking for

Might I recommend Dungeons & Dragons Online (DDO)? It is what WoW should have been if they could have bothered to negotiate the license. It is also free to play far enough to find out whether you like it enough to spend some money on it. :)

There is also Lord of the Rings Online (LotRO). It is based on the books, and the world creation is just beautiful. You can play that game in many different ways, from slaying your way to glory to hanging out with friends and playing music (there is an internal music system and instruments with which to play).

you can pick up speedrunners for 1 dollar in the tiny build bundle from humblebundle.com it's simple and super fun and for that price i'm sure you won't regret it

Terraria
saint's row 3 &4
victor vran
path of exile
torchlight 2
grim dawn
borderlands series
binding of isaac rebirth
quake (?)
guild wars 2
maplestory 2
emulated nes games (there 's plenty you can do co op , if you like nes games)
sonic allstars racing
skyrim's co op mod (last i heard that was a thing, i'm not sure about now if it still works or nah)
portal 1 and 2
left for dead 2
payday the heist (1 and/or 2)

There's quite a few games that is co-op or multi you can play for free during the weekend in the Spring Cleaning Event Have fun over the Weekend! Cheers~