It is within the/a religion's doctrine-

a Jew must/should marry a Jew

a catholic must/should marry a catholic

a Christian must/should marry a Christian

a Muslim must/should marry a Muslim

(pardon my ignorance but i don't know about Hindus, and not that sure about Buddhists either...)

but-

it is not a 'must' or 'should' that a free-thinker marry another free-thinker.

but a free-thinker cannot marry a jew, catholic, christian or muslim,

not because free-thinkers don't want to,

but because jews, catholics, christians and/or muslims don't want to/it is not allowed.

so free-thinkers end up marrying free-thinkers?

in no way is this post meant to offend anyone or any religion,

just a thought, an observation.