Nikah (marriage) is a Sunnah and a natural need as well. Islam teaches us to have high morals and pure characters. To maintain relation with anyone before marriage is against Islam and against morality. After marriage, husband and wife should live together with love and affection. If love marriage means that after marriage, there should be love between the husband and the wife, then it is appreciable. There is no concept of love before marriage, it is only a different name for moral degeneracy. No person will accept the same for his sister. Before marriage, one may see the girl once.
Marriage with cousin
As per the Shariah, marriage with cousins is permissible and allowed. After describing the Mahram relatives (with whom marriage is impermissible) Allah Ta'ala says:
………thus hath Allah ordained (prohibitions) against you: except for these, all others are lawful. Surah Nisa (4:24)
Marriage is permissible with relatives who are not included in the Mahram relatives. Thus, marriage with them is permissible.
Consent of the girl is also necessary for marriage
Islam regards the consent of the girl also as necessary. The consent of an adult and sane girl is also necessary for marriage.
However, if a girl wants to marry someone below her (Gair-Kufu), then the parents can stop her and still if she ends up marrying, then they can get the marriage annulled. Also, if the Mehr (alimony) is less than "Mehr Mithl" (the alimony of her mother/aunt), then the parents/guardian can demand it. They can also demand a separation as given in Fatawa A'alamgiri, Vol. 1, Pg No. 292/293/294.
Demanding dowry in marriage
Translation of Hadith: There is a very lengthy Hadith narrated on the authority of Hadhrat Habshi Bin Janadah As Saluli in which the Holy Prophet (Sallallahu alaihi wa sallam) said: Begging is not permissible for some who is wealthy and someone with a sound body, excepting for someone who is very poor and someone in a very great need. If a person begs to increase wealth, then in Qiyamah, that begging will be on his face in the form of scratches and it will come in the form of an ember, from which that person will eat. Whoever wants to can decrease or increase this punishment for himself.
(Jame' Tirmidhi, Kitab Uz Zakaah, Vol. 1, Pg No. 141, Hadith No: 590)
However, without any demand of any kind, exchanging gifts willingly is allowed. Islam has declared exchange of gifts to be a means of maintaining and increasing love, as the Holy Prophet (Sallallahu alaihi wa sallam) said in Kanz Ul Ummal, Hadith No: 15057). Thus, giving a gift on the occasion of marriage is permissible.
If someone has been a Qadiani before marriage then the marriage itself is not valid and as such, the couple are not husband and wife and should immediately separate. As given in Durre Mukhtaar, Babi Nikahil Kafir and Kanz Ud Daqaaiq, Babi Nikahil Kafir.
Maintaining justice between more than 1 wife
In the Shariah, it is the responsibility of the person who marries more than once to be equally just with both wives. Even in the fulfillment of conjugal rights, this equality has to be maintained. If the husband spends one night with one wife, then he should spend one night with the other also. If 2 nights with one wife, then the same number of nights with the other. If he cannot maintain justice between 2 wives, then on the Day of Judgment, one of his hands will be cut. As given in this Hadith:
Translation of Hadith: It has been narrated on the authority of Hadhrat Abu Hurairah (May Allah be well pleased with him) that the Holy Prophet (Sallallahu alaihi wa sallam) said: If a person who has 2 wives and he does not maintain justice between them, then in Qiyamah, he will come with one of his hands cut.
(Jame' Tirmidhi, Hadith No: 1171)
Marriage with a Non-Muslim
As per the Shariah, marriage with a non-Muslim is forbidden. If the girl accepts Islam sincerely and testifies that Allah (Subhanahu wa Ta’ala) is the One and only Lord and Hadhrat Muhammad (Sallallahu alaihi wa sallam) is His Last Messenger, then marriage with the girl is permissible. For the marriage, it is necessary for 2 sane, grown up Muslims to witness it. If the girl embraces Islam, marriage with her is allowed. Still, you should give importance to the opinion of your parents also. The pleasure of Allah (Subhanahu Wa Ta'ala) lies in the pleasure of parents and your life will become blessed because of it.
Marriage with a Qadiani
If the couple were both Muslims before marriage and after marriage, either of them became a Qadiani, then immediately the marriage is dissolved. As given in Durre Mukhtaar, Babi Nikahil Kafir, Hidayah, Kitab Un Nikah, Babi Nikah Ahle shirk.
The children will be considered to be Muslims after the parent who is a Muslim. As given in Radd Ul Muhtaar, Babi Nikahil Kafir.
If either of the parents becomes Qadiani, then their parentage is not nullified and the children will be considered to be their children only. There are many examples in the era of the Sahabah that in spite of their fathers still being non-Muslim, their parentage was not negated.