A) The bridegroom/husband is parameshwar for the bride/wife. The husband is the supreme Lord for the wife. The Hindu traditional stri-dharma husband is GOD, as followed by Sita, Nalayini, Savitri,Anasuya, Ahalya ....
One of the Hindu marriage rituals is Kanyaa Daanam:
"Mahavishnu swarupasya varasya etham asanam"
"Mahavishnu rupa vara swaghtam, ethamthey pathyam"
Here, the brahmachari meets his prospective father-in-law. The latter seats him facing the eastern direction and washes the feet of the future son-in-law, considering him as Lord Vishnu Himself. All honors are given including the ceremonial washing of the feet of the groom by the father-in-law and offer of madhu parka (a mixture of yogurt, honey and ghee) to the accompaniment of selected Veda mantras.
B) For better understanding the question is rephrased as follows:
Why is Parameshwar called Pati Parameshwar?
Simple meaning is Parvathi's Pati. But the term is more than that. Here the term 'Pati Parameshwar' is 'Supreme Godly Husband.' Then it is not simple as pati and patni (Male and female relationship as husband and wife). It is on Pati, Pasu and Pasam in Saiva Sidhanta (Shiva Worship).
Saiva Siddhanta postulates the existence of three ultimate entities of realities. These are Pati or Lord Siva (Parameshwar); Pasu or Soul, Pasam or the bondage of souls. These three are eternal and have neither beginning nor end.
Pati or Lord Parameshwar is in its nature spiritual in form and is Almighty, All-merciful, Omnipresent, just and perfect and all love and ever blissful. He is beyond the approach of Pasu and He is the fountainhead of eternal happiness when there is on return, when once reached.
Pasus (Souls) are infinite in number. They are of crystal-like nature ready to reflect the object before which they are placed. They are also spiritual in form and are capable of enjoying the eternal happiness as recipients of the grace of God. But, they are enveloped in Pasam and hence subjected to pain and sorrow.
Pasam or the bondage for souls is divided into three kinds Anavam, Mayai and Kanmam, which are known also as the three malas. Anavam makes the soul ignorant and arrogant and it is egoism. Mayai is the source of material universe and capable of lifting man from his dormant and inert nature with the help of Pati, Kanmam is the accumulation of good and bad actions of souls and the cause of births and deaths of mankind. Pasu and Pasam are so closely intimate that we cannot see the one separately from the other. Yet, their union is not indissoluble. Pasu or soul will be released from the bondage of Pasam by the grace of Pati and when the posers of Pasam are exhausted, the Pasu or soul can clearly see Pati and be one with Him to enjoy the heavenly beatitude. Just as the soul had an inseparable union with Pasam in its original (Kevala) state, so also it will have the same inseparable state with Pati, after it is fully released from the bondage of Pasam. This relation is called ‘Adwaitam’ viz., unity in duality.
Reference 2: Pasu-pati is Parmeshwar:
According to the followers of Saivism, Siva is the pati, the husband, while jivas are pasus or animals or beings that have lost sight of their absolute nature because of atomicity or their belief that they are limited, finite beings who are separate and different from God. Parameshwar is pasupati or the lord of all animals or jivas. As pati, he is the absolute principle, the one undivided reality, the effective cause of all creation.
I agree with Sri.Chandrasekaran sir for the common meaning of 'Pati Parameshwar' as 'husband is the supreme Lord for the wife'. But the term 'Pati Parameshwar' is 'Supreme Godly Husband.' (Reference 3).
Also The Pati, Pasu and Pasam consept is not confined only in Sothern India or in one philosophical domain but also in Northern part and many philosophical domains. The Pasupatinath temple is in Kathmandu, Nepal.
Followers of Vishishtadvaita school of Saivism believe that Siva and Jiva are one and the same reality. At the end of creation, all the jivas return to Siva and he alone exists. Followers of Siddha school of Saivism do not agree with this view. Even after self-realization, the soul remains distinct from Siva and does not dissolve into him. (Reference 2)
Kashmiri Saivism also goes by other names such as Trika Saivism, Spanda Saivism and Pratyabhigna Saivism. These three represent three distinct approaches or perspectives within Kashmiri Saivism without disagreeing on the fundamental concepts such as the unity of the soul and Siva. Trika Saivism emphasizes the three main principles of, pati, pasu and pasa or Siva, Shakti and anu, as on reality. Spanda Saivism refers to the dynamic power (Shakti) of Siva, or the first impulse (spanda), which is responsible for the manifestation of the pluralistic worlds. Pratyabhigna Saivism refers to the realization by an individual soul of its true identity with Lord Siva. So liberation of a soul is not dissolution but plain liberation from the impurities that hold it in check.
An interesting paragraph appeared in an article on Sikh marriage in Panthic Weekly dated, 22nd may 2005:
'Four hymns (called laavan) from the Guru Granth Sahib Ji are read to solemnise the Anand Kaaraj (Sikh marriage). These hymns are enshrined in the Guru Granth Sahib at pages 773-74. In these hymns, Guru Raam Daas Ji, the fourth Sikh Guru has written about the marriage of the individual Aatma (soul) with Parmaatma (the Eternal Soul). These four hymns mention four stages in the progression of love between spouses and also of human souls towards union with the Supreme Soul.'
Another paragraph appeared in Letters to the Editor coloumn in connection with the above article, in the same journal dated, 5th of June 2005:
'I was married under this ceremony so let me tell you I believe in Guru Granth Sahib and all the Gurbaani. In the Laava(n), Guru Ram Daas Ji talks about the meeting of Aatma and Parmaatma, Calls The God 'Pati Parmeshwar' but Sikhs have attributed all the qualities of 'Pati Parmeshwar' to the Sikh bridegroom. True enough, the proper respect is applied to Guru Granth Sahib but when Laava(n) are performed it is implied that man being married is the 'Pati Parmeshwar' and the woman is the lower being only a human soul.'
The author/editor gave the reply as follows:
'Pati Parmeshwar' has not been attributed to the bridegroom by Sikhs or the Guru Sahibaans. 'Pati Parmeshwar' is a Indian concept and tradition which has been used as metaphor by Guru Sahib to describe the love and dedication between the human soul and Vaheguru (the Wondrous Lord).
In Sikhi man and woman are equal in status.
Anyone who believes man is greater than women or believes that a wife should regard and serve her husband as a "god" and that the wife is mere a slave of the husband is mistaken and ignorant of Gurbaani and Gurmat. There is nothing objectionable with a wife serving her husband as a 'god', but at the same time the husband should then serve and treat his wife as a "Patni Devi", a goddess.