To argue this, we must say that the marketing of fast foods makes people more likely to eat foods that are bad for them and which tend to cause obesity.
From this point of view, marketing of fast foods makes them seem more attractive to consumers. This can be particularly true if the marketing focuses on children, encouraging them to push for visits to fast food outlets. When marketing does this, it increases the likelihood that people will eat at fast food restaurants. When people do this, they are more likely to eat fatty and sugary foods in large portions.
We can argue, then, that marketing makes people more likely to want to eat food that is unhealthy. In this way, it contributes to obesity.