Answer
Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Dear questioner, we commend your pursuit of knowledge and your eagerness to seek what is lawful and avoid what is not. We earnestly implore Allah to bless your efforts in this honorable way.
Generally speaking, a valid marriage has to meet certain requirements such as announcement, the payment of the dower, the consent of both parties, the permission of the wali (woman’s guardian), and the presence of witnesses.
As for a civil marriage which is conducted in Western countries, it should be noted that if the marriage contract contains all the conditions and there is no Shari`ah reason to prevent it, then it is valid, which makes it permissible for the couple to enjoy their conjugal rights. However, if one of the couple is from a Muslim country then the contract must be held in a Muslim country [or registered with the appropriate consulate(s)] so that the rights of both partners will be preserved according to the Shari`ah.
In this regard, Sheikh Faysal Mawlawi, the late Deputy Chairman of the European Council for Fatwa and Research, states:
The legal marriage which is done in European countries is regarded as a valid marriage in the Shari`ah as long as there is no legal reason in the Shari`ah against it. This is because the basic condition in a valid marriage is the agreement of the couple, which is present in case of the civil marriage or court marriage. Also, announcement is usually implied. As for the condition of two witnesses it is, according to some schools, required to guarantee that the marriage is announced, and this is achieved in a legal marriage. As for the agreement of the guardian, it is an issue of disagreement among schools of Fiqh.
Therefore, the only condition which invalidates a court or legal marriage in the West is if there exists any reason which prevents marriage in the Shari`ah. For instance, a Muslim is not allowed to marry his foster-sister, though this is permissible in European laws. (Likewise, a Muslim woman is not allowed to marry a non-Muslim and a Muslim man is not allowed to marry any non-Muslim other than a Christian or Jewish woman, though all types of inter-faith marriage are permitted in Western laws.) Therefore, if a marriage is contracted between a Muslim and his foster-sister in a Western country, this marriage is not valid in Shari`ah as there is a reason which prevents marriage and it is haram for the concerned partners to engage in sexual intercourse.
If there is no Shari`ah reason to prevent the marriage, then the marriage done in a Western country can be regarded as a valid marriage in the eyes of the Shari`ah and can be accepted by the Shari`ah courts in Muslim countries. This is the rule, particularly if the Muslim couples are of a European nationality or origin. However, if one or both of them hold nationality of a Muslim country, then they must make the marriage contract in a Muslim country [or register it with the appropriate consulate(s) so that the marriage is recognized abroad]. If, however, they make the contract in a non-Muslim country, then the marriage is still valid as long as there is no Shari`ah reason to prevent the marriage, yet they are sinful for not resorting to a Muslim court and accepting to apply non-Muslim laws instead.
As for the Shari`ah reasons that prevent marriage, they are as follows:
1- If the wife is one of the prohibited categories to marry in the Shari`ah including mother, daughter, sister, paternal and maternal aunts.
2- Foster-mother and foster-sister.
3- A Muslim man is not allowed to marry a married woman or a widowed or divorced woman who is still in her `iddah(waiting period) after being widowed or divorced. Only after the elapse of `iddah, marriage is permissible.
4- A Muslim is not allowed to have in marriage more than four wives at the same time, or to have two sisters in marriage at the same time, or to have a woman and her paternal or maternal aunt at the same time.
5- A Muslim woman is not allowed to marry a non-Muslim, and a Muslim man is not allowed to marry any non-Muslim other than a Christian or Jewish woman.
Allah Almighty knows best.