Since you are writing a Catholic website, I’m assuming that either you or your fiancé is Catholic, so I’m going to answer the question with that in mind. You also don’t mention whether the proposed marriage is between two Christians or between a Catholic and someone from another religion, so I’ll answer both.
It’s important to remember that strictly speaking, the Church prohibits mixed marriages (between a Catholic and a Christian of another tradition, also called “interchurch marriages”) and interreligious marriages (between a Catholic and an unbaptized person). Dispensation from this prohibition by the competent diocesan authority is possible, but is certainly not guaranteed (it certainly happens often enough, but indeed there are lots of paperwork and discussion involved). This is because of the very significant challenges facing people in such marriages, including what to do about the religious education of children. Experience has shown this to be a major point of conflict if it is not resolved before the marriage takes place.
As part of the application for a dispensation for an interchurch or interreligious marriage, the Catholic party must make a solemn promise to remain active in their Catholic faith. They also pledge to do all in their power to have the children baptized in the Catholic Church and to raise them in the Catholic faith. Accordingly, the non-Catholic fiancé must be informed of the Catholic party’s obligations in this regard. If he or she has serious reservations about their Catholic fiancé’s solemn obligation to raise the children in the Catholic Faith, then it is vitally important that the matter be resolved before they continue with marriage preparation.