Muslims:
Heresies of same faith: moderate reluctance
Shia to Sunni and vice versa (including heresies): strong reluctance
Christians and Zoroasterians: strong reluctance
Pagans: extream reluctance / impossible
Muslims should have no problem marrying infidel women (it happened on a routine basis), but be strongly reluctant (if not outright unwilling) to marry their women to infidel/heretical men. Alfonso VI took a Muslim mistress (it's unclear whether Zaida of Seville actually married him or not), but even she was a refugee in his court and probably a convert.
Anyway, +1 to the idea. There were plenty of mixed marriages when political concerns trumped religion.
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