If ALL you can find is pizza, pasta, and beef, you are not looking very hard.
Me, I eat lots of fruits and vegetables, salads, and pork, chicken, and, yes, seafood.
Lots of good peruvian food- clusters of Peruvian places around Abasto.
Which feature stews, vegetables, and seafood.
A fair amount of quite good modern asian/argentinan fusion food out there too- Sudestata, in Palermo Hollywood, does decent pomo stir fries, at a good price. I like Sifones y Dragones, one of the Puertas Cerradas restaurants which are quite popular here-
see Dan Perleman's excellent blog for a rundown on these- he doesnt like Sifones, but I have had great meals there.
http://www.saltshaker.net/underground-dining-scene
I find plenty of non-carne dishes to love in BsAs. Omlettes and soups, mushrooms and asparagus, fruits in season, liquados and grilled veggies.
And I cook myself, especially Thai, Asian, and Mexican, which I find disapointing in BsAs,and one of the first things I bought was a blender, as I like my fruit liquados with no sugar, and lots of grated ginger.
I am a big fan of the family style restaurants, which you generally find outside of the Tourist Crescent, where the "salad bar" is huge, and contains tens of prepared salads, ingredients, and great veggies, all you can eat at a low price. I was in one a while ago just off Corrientes on Avenida de Estado de Israel, where the salad bar would satisfy anyone, with so many choices. Artichokes, beans, potatoes in many forms, fresh greens of many types- and just a few pesos, with free refills.
Again, you gotta EAT LOCAL.
Its all out there, if you are not restricted by your preconceptions.