In this sentence, marrying acts as the object of the verb "contemplated." Since it is a verb ending in -ing that functions as a noun, it is a gerund.

১. A present participle usually acts as an adjective or forms continuous tenses.
২. An infinitive starts with "to" (e.g., "to marry"), which is not the case here.
৩. Certain verbs like "contemplate," "enjoy," and "avoid" are always followed by a gerund.

অতিরিক্ত তথ্য: Gerunds are often called "verbal nouns" because they have properties of both.