Additional Shia doctrines
There are additional Shia doctrines besides the Theology of Twelvers and the Aspects of the Religion.
Taqiyya
The doctrine of taqiyya, or dissimulation, states that it is permissible to hide one's true religious convictions if under the threat of death or injury. Since Shias regard taqiyya as Islamically permissible, at times they have been accused of lying indiscriminately, however Shia Muslims reject this charge and argue that this would be contrary to the laws regulating its use. The Shia conclude that the practice of taqiyya is condoned by the Qur'an and is regulated by the various Qur'anic verses that refer to it. The practice of taqiyya is regarded by the Shia as a natural human response especially in light of historical Shia-Sunni relations.
Nikah Mut'ah (Mut'ah marriage)
Fixed time marriage (Arabic: Nikah Mut'ah) is the second Qur'anic marriage form, a marriage for a fixed time with fixed conditions stipulated through mutual written or oral contractual agreement between the male and female participants.
The practice was instituted by the Islamic prophet Muhammad and sanctioned in the Qur'an. Shia conclud it is allowed according to Sharia. In contrast to non-Shia Muslims, Shia Muslims conclude that Mut'ah marriage was only forbidden by the caliph Umar and not by Muhammad. Shia argue that neither Umar, nor any other caliph, had the authority to ban what Muhammad permitted, so Shias judge its practice to remain legitimate.
By far the most common form of marriage among Shi'as is the Nikah (regular marriage), and abusing Mut'ah marriage to lead a promiscuous life is frowned at by Shias, quoting the Qur'anic call for chastity. The most common use of it is by couples that intend to marry permanently in the near future, something akin to the Western practice of engagement.
Defending Nikah Mut'ah
Criticizing the Shia conclusion regarding the validity of Nikah Mut'ah is one of the main things done by Sunnis in Shi'a-Sunni argumentations.
The practice of Nikah Mut'ah is limited, and many Shia would not be affected by a change of policy regarding its legality. However, Shi'a Muslims defend its legitimacy, not due to any broad use of it, rather since they truly believe it was made legal by the Qur'an and Sunnah and that it remains legalised, hence they believe any proclamation (subsequent to the passing of Muhammad) that attempts to make it illegal is inconsistent with the Qur'an and Sunna and hence would be considered a major sin. Shi'a believe they have a strong and solid case regarding the validity of Nikah Mut'ah and present various arguments in support of its legality.
Shia Muslims are accustomed to refuting the moral accusations of the practice, and also show as proof how some of the Sahaba treated the subject and practiced it. Many Shias feel wary of the controversy surrounding the subject and feel that it is blown out of proportion, since they feel being a Shia is much more than a debate regarding Nikah Mut'ah being legal or not.