I would agree with you, but I notice that Sofia easily resorts to deception in order to keep the truth secret. For example, showering at school with a janitor doesn't tell Liam anything about it. When Sofia accepts Aiden's blackmail, she says nothing about it to Alissa. Leaving to Aiden, while trying on a wedding dress, he also easily deceives everyone saying that he is going to school.
And she also accepts extreme situations for herself if she is with strangers, people far from her family circle. Therefore, I believe that Sofia's first extramarital affair will be with a complete stranger. It will be an outburst of her emotions, sexuality, probably as a result of dissatisfaction, fatigue and a simple manifestation of weakness.