Since the sex was apparently consensual, I'd say the odds are that the girl thought she was in love with the guy.
Whether or not she will maintain that feeling over years of marriage is another story. On an anectdotal level, I know of one couple that had to get married in my high school as freshmen have had a great marriage for more than fifty years. They will tell you today that they were more determined to make a go of it because of all the naysayers who said it would never last.
When you think about it though, it makes sense that a young marriage would last. Marriage is a learning experience and we are much better at learning when we are young.
Older people tend to get set in their ways and are less likely to compromise with their spouse.
Back in '61 or '62, my 18 yr old cousin was sneaking around with a14 yr old girl. They got "caught" when she got pregnant and had to tell her folks.
Her dad put her, her 2 brothers and his shotgun in the car. He drove to the courthouse and got them a marriage license and went by the parsonage and picked up the preacher. Drove straight to my cousin's house and called him outside.
Told him "son, you have got this girl in a family way and today is either going to be your wedding day or your dying day. Whichever one you chose, you are gonna need that preacher"!!!
They got married right then and there in the front yard. Her daddy told my cousin "she belongs to you now and you had better take good care of her and that baby" and drove off.
They are still married to this very day. When the best man at your wedding is a 12 ga. shotgun, I guess it is a good idea to stay married.