From this answer:
अतीते द्वादशाब्दे तु दृष्ट्रा तां नवयौवनाम्। सार्धं रायणावेशयेन तत्संबन्धं चकार सः॥ ३८ ।।
After the expiry of twelve years, finding her becoming youthful, she was married to a trader named Rayana
According to social standard, yes, it can be considered an extra marital affair.
Radha's depictions vary from being an already married woman who becomes an adulterous lover of Krishna in a secondary role, ... In some Hindu sub-traditions, Radha is conceptualized as a goddess who breaks social norms by leaving her marriage, and entering into a relationship with Krishna to pursue her love. [Wikipedia]