Translation:My husband never sleeps on the sofa, but I do.
I HAD TO LOOK THIS UP!
Apparently this is a "common" expression, idiom or other. http://www.linguee.com/english-spanish/search?source=autoquery=yo+s%ED
Here's one of the examples:
Yo sí creo en la calidad del tiempo que uno da
I believe in the quality of the time that you give
I am starting to be concerned about these people. When we started out, the were all drinking milk and eating cheese. Life was fine. But now we find they are sleeping on the sofa and commenting that she is as pretty as her older sister. What could possibly be next? Is duolingo.com actually a stepping stone to novelas on Telemundo?
Be careful about the accents. "sí" means "yes", or "yourself" but "si" means "if". "sí" and "si" are two completely different words. You could think about it in English with the words "meet" and "meat", which sound the same but have two completely different spellings, and two completely different meanings.
I find what works for me is to think about 'sí' as an affirmation. Even thinking about the English word 'yes' in this way makes sense. "My husband never sleeps on the sofa, but I say yes to it (I do)." or "I say yes to (I believe in) the quality of the time that you give."