For it is written, "If any one thinks that he is not behaving proper toward his betrothed, if his passions are strong, and it has to be, let him do as he wishes: let them marry-it is not a sin (1 Corinthians 7:36)." And here the betrothed may be a believer or an unbeliever.
Someone can be betrothed to you at a very young age when he/she knows nothing. This tradition is common in Israel, in the Middles East and in Africa in those days. And it is still practice today. Now, you can see that the betrothed may be a believer or an unbeliever.