https://daruliftaa.com/nikah-marriage/a-specific-case-of-sunni-shia-marriage/
[TABLE="class: grid, width: 100%, align: center"]
[TR][TD]
A Specific Case of Sunni-Shi'a Marriage
If it is true that he does not hold any beliefs that constitute disbelief (kufr), which include (and is not limited to) believing that the Qur'an was altered, accusing Sayyida A'isha (Allah be pleased with her) of committing adultery, denying the Companionship (suhba) of Sayyiduna Abu Bakr (Allah be pleased with him), and the other things you have mentioned in your question, then technically your marriage with him will be 'valid', meaning that the marriage contract will be recognized in Sunni law, and you will not be guilty of living in a unlawful relationship. However, if he does hold beliefs that constitute Kufr, then the marriage will not be considered valid; hence, even if you were to marry him, you will be considered living in an illicit unlawful (haram) relationship. This was explained in detail in a previous answer posted on our website titled: 'Marriage with a Shi'a'. The Sunni position on marrying a member of the Shi'a community who does not hold beliefs that constitute Kufr is that even though the marriage may 'technically' be valid, it is still best avoided due to the many stances of Shi'as being considered as deviation by Sunni Muslims. The gravity or otherwise of taking this step also depends on the exact nature of beliefs held by the person in question. You state that the brother whom you wish to marry believes that 'Hazrat Ali should have been the first caliph' but 'he has never shown any disrespect for the other three Caliphs or Sahaba'. This is very significant. If you can be fully assured and convinced of the fact that he has the utmost of respect and regard for all the Companions (Allah be pleased with them all), and that he considers the other three Caliphs to have a higher status than Sayyiduna Ali (Allah be pleased with him) in the order of: 1) Abu Bakr, 2) Umar, 3) Uthman, 4) Ali (Allah be pleased with them all), then I believe you may marry him. In other words, the only differences between him and Sunni Muslims are the following: 1) Sayyiduna Ali (Allah be pleased with him) should have been the first Caliph of the Muslims, but this does not mean any of the other Companions (sahaba) were guilty of any wrongdoing, rather they are all forgiven by Allah, and Allah is pleased with all of them and they are all pleased with Allah. They are the highest of people after the Messenger of Allah (Allah bless him & give him peace), and the highest in Maqam is Abu Bakr, then Umar, then Uthman and then Ali (Allah be pleased with them all). 2) He attends lectures on the topic of the martyrdom of Sayyiduna Husayn (Allah be pleased with him) in the month of Muharram, but does not believe in self beating, etc. Other than the above two things, if there are no issues, then you may go ahead and marry him, but it will still be somewhat disliked to do so. The reason for this is that according to the beliefs of Sunni Muslims, one who believes that Sayyiduna Ali (Allah be pleased with him) was the worthy first Caliph is also misguided. (See: Al-Aqida al-Tahawiyya with its commentaries) As such, marriage should be avoided in normal situations and whenever possible. Another aspect worth considering is whether you will be free to practise Islam as a Sunni, and whether your children will also have a chance to be Sunni Muslims. And Allah knows best...[/TD][/TR][/TABLE]