[Canticle_of_Deborah]

Why are people desiring conversion held to Catholic doctrines on marriage when they were not Catholic nor married Catholic in the first place?

[Tantumergo]

"Why are people desiring conversion held to Catholic doctrines on marriage when they were not Catholic nor married Catholic in the first place?"

Because they are not just "Catholic doctrines" on marriage - it is Divine Law and it is immutable.

The Church recognises the validity of marriages among pagans, because although they are not Sacramental marriages, they are covenantal marriages in the eyes of God according to the Natural Law.

The covenant between man and woman in marriage is the most fundamental covenant after God's covenant with creation itself (hence the Genesis account where both covenants are related side by side in the creation narrative) and hence can never be taken lightly:

"What God has joined together, let no man put asunder!"

Nevertheless there are annulment possibilities for non-Catholics becoming Catholic, which are not options for Catholics. Decisions may be arrived at "In favour of the Faith".