Translation:We are washing the dog because it stinks.
Out of curiosity:
I notice that dogs in this course are referred to by the pronoun se. Is hän reserved for humans? It used to be this way in English, but nowadays animals, particularly pets, are referred to as "he/she", and certain pet owners will get offended if you use "it". Based on the number of lessons about pets, I am going to assume that at least one of the course contributors is a pet owner. :) Can any of our suomalaiset shed some light?
No. The use of the partitive object koiraa implies an ongoing process, which requires present progressive rather than simple present. If you want to use simple present to imply a habitual practice, like washing the dog weekly, the object would be in the accusative case, koiran. Ex:
"Mitä teet lauantaina? Minä pesen koiran."