Answer: [fatawa riDawiyyah, kitab an-nikah; vol.11/pg345, Lahore edition]
Shi`ah fall into three categories:
1. ghâli (ghulât): they repudiate the necessities of religion. for example:
a) to believe that qur'ân is incomplete.
b) to call it 'book of `uthmân'.
c) elevate sayyiduna `ali karram Allâhu wajhah and other imâms above the prophets salawâtAllâhi `alayhim wat taslîm.
d) if these imâms are held to be higher than even ONE prophet.
e) to allege that Allâh `azza wa jall was regretful after issuing a command and hence remorsefully, changed His earlier ruling
f) to allege that Allâh didn't realize the wisdom of a certain ruling (or the lack of it) and when He realized it, He changed the rule.
g) to allege that RasûlAllâh SallAllâhu `alayhi wa sallam practised taqiyyah in the course of his tabligh
those who hold the above and other such statements that amount to disbelief are kâfirs by ijmâ`a. all dealings with them are similar to those with apostates. it is in fatâwâ DHahîriyyah, fatâwâ hindiyyah, Hadiqatun Nadiyyah: [aHkâmuhum aHkâm al-murtaddîn] they are to be dealt with as apostates.
nowadays, most of the rafiDis fall into this category. their scholars and commoners, men and woman all of them seem to profess the aforementioned beliefs, except Allâh willing otherwise.
a woman with such beliefs is an apostate. a Muslim cannot marry her, nor a kafir nor an apostate. if a Muslim marries her and has intercourse, it is fornication. and a child born of such a relation is illegal and considered as a bastard.
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2. tabarrâyi (sabbiyyah) : they don't profess such egregious beliefs but indulge in abusing the SaHâbah.
some people among this group reject the khilâfah of the shaykhayn raDiyAllâhu `anhumâ. these people are considered kâfir among the fuqahâ as is evident from khulâSah, hindiyyah etc. *BUT* the correct ruling is that of the mutakallimîn and that is they are bid`yis – subject to hell and dogs of hell – but not kâfirs. to marry a woman who belongs to this group, is valid but is profusely disliked. because it is from the Hadîth 'laa tanâkihûhum' / don't marry the people of bid`ah.
it is in SaHîH Hadîth that a man cursed a she-camel. RasûlAllâh SallAllâhu `alayhi wa sallam told them to leave the she-camel since it had become accursed. after this nobody ever touched that she camel (muslim).
it should be noted that a she-camel, is in no way liable to be cursed. it doesn't deserve to be cursed. but a tabarrayi rafiDi who curses the shaykhayn is himself cursed by Allâh. "these are the people who are damned by Allâh, and the damnation of the damners upon them" (2:159)
hence it is not desirable to live with a woman who is accursed (because of her cursing the shaykhayn).
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3. tafDîli (tafDîlîyyah): this group revers all the companions; the accept the leadership of all the four Caliphs raDiyAllâhu ta`âlâ `anhum; but just elevate the position of `Ali raDiyAllahu `anhu above all others.
these people are nowhere near kufr. however it is bid`ah to do so (consider `Ali higher than others).
to marry such a woman is permitted (without any difference among fuqaha), yet it is disliked. if being with her may influence one's opinion of ahlus-sunnah, the dislikeability would increase to the point of prohibitively disliked (makrûh taHrîmi).
And Allâh sub-Hânahu wa ta`âlâ knows best.
Fatwa Radhiwiya, Vol 13 , Kitab al Talaq)