What motivates a fully awakened person in a loving relationship? Loving relationship here is meant specifically as a mutually intimate partnership like husband and wife and boyfriend and girlfriend. There would be the case where one would become fully awake and therefore abandons all teachings but whilst still in a relationship. Would this change the dynamics of that relationship in some major way?

What do the Suttas say about this if anything?
Remark: This is not (yet?) a full answer, but more an extended comment.          

For one impression of how might a loving relation be revered see  "A.IV. 55 Nakulapitá und Nakulamátá" ; although the couple is not called arahants they are mentioned as "top on lay disciples" (see "A I.24")  "The foremost of my laymen ...(...) ... who are intimate is the householder Nakula’s father.”
So there's no right-out disregard of being a loving couple.          

Next it might be interesting a story about the (later) arahant (Maha) Kassapa about the problem of being married and having a desire for ascetic life at the same time. Hellmuth Hecker has compiled a story about this which I found some time ago at the access-to-insight site (I think) . Here is a part of that text on the biographic tale about Kassapa:                 


  (...) Like the two chief disciples, Sariputta and Maha Moggallana, Maha
  Kassapa too descended from the brahmin caste, and again like them, he
  was older than the Buddha. He was born in the Magadha country, in the
  village Mahatittha, as the son of the brahmin Kapila and his wife
  Su-manadevi.         
  
  (This account of Maha Kassapa's early life is taken from the commentary to the Samyutta Nikaya.)      
  
  He was called Pipphali. His father owned sixteen villages over which he ruled like a
  little king, so Pipphali grew up in the midst of wealth and luxury.
  Yet already in his young years there was in him the wish to leave the
  worldly life behind, and hence he did not want to marry. When his
  parents repeatedly urged him to take a wife, he told them that he
  would look after them as long as they live, but that after their
  deaths he wanted to become an ascetic. Yet they insisted again and
  again that he take a wife, so to comfort his mother he finally agreed
  to marry -- on the condition that a girl could be found who conformed
  to his idea of perfection. For that purpose he shaped a golden statue
  of a beautiful woman, had it bedecked with fine garments and
  orna-ments, and showed it to his parents, saying: "If you can find a
  woman like this for me, I shall remain in the home life."          
  
  His parents approached eight brahmins, showered them with rich gifts,
  and asked them to take the image with them and travel around in search
  of a human likeness of it. The brahmins thought: "Let us first go to
  the Madda country, which is, as it were, a gold mine of beautiful
  women." There they found at Sagala a girl whose beauty equaled that of
  the image. She was Bhadda Kapilani, a wealthy brahmin's daughter, aged
  sixteen, four years younger than Pipphali Kassapa.         
  
  Her parents agreed to the marriage proposal, and the brahmins returned to
  tell of their success.          
  
  Yet Bhadda Kapilani also did not wish to marry, as it was her wish,
  too, to live a religious life as a female
  ascetic. Such identity between her aspiration and Pipphali Kassapa's
  may well point to a kammic bond and affinity between them in the past,
  maturing in their present life and leading to a decisive meeting
  between them and a still more decisive separation later on.      
  
  When Pipphali heard that what he had thought most unlikely had actually
  occurred, he was -- unhappy and sent the following letter to the girl:
  "Bhadda, please marry someone else of equal status and live a happy
  home life with him. As for myself, I shall become an ascetic. Please
  do not have regrets."
  Bhadda Kapilani, like-minded as she was,
  independently sent him a similar letter.
  But their parents,
  suspecting such an exchange would take place, had both letters
  intercepted on the way and replaced by letters of welcome. So Bhadda
  was taken to Magadha and the young couple were married. However, in
  accordance with their ascetic yearning, both agreed to maintain a life
  of celibacy. To give expression to their resolve, they would lay a
  garland of flowers between them before they went to bed, determined
  not to yield to sensual desire. This young wealthy couple lived thus
  happily and in comfort for many years. As long as Pipphali's parents
  lived, they did not even have to look after the estate's farms. But
  when his parents died, they took charge of the large property.
  (...)    


Now,  further after the death of Pipphali's parents, Pipphali and also Baddha took the yellow robe and left home, but walked together. Then this happened:


  (...) When walking on, Kassapa went ahead while Bhadda followed behind
  him.    Considering this, Kassapa thought: "Now, this Bhadda Kapilani
  follows    me close behind, and she is a woman of great beauty. Some
  people -    could easily think, 'Though they are ascetics, they still
  cannot live    without each other! It is unseemly what they are
  doing.' If they spoil    their minds by such wrong thoughts or even
  spread false rumors, they    will cause harm to themselves." So he
  thought it better that they    separate. When they reached a
  crossroads Kassapa said: "Bhadda, you    take one of these roads, and
  I shall go the other way." She said: "It    is true, for ascetics a
  woman is an obstacle. People might think and    speak badly about us.
  So please go your own way, and we shall now    part."
  She then
  respectfully circumambulated him thrice, saluted him    at his feet,
  and with folded hands she spoke: "Our close companionship    and
  friendship that had lasted for an unfathomable past comes to    an
  end today. Please take the path to the right and I shall take the 
  other road." Thus they parted and went their individual ways, seeking 
  the high goal of Arahatship, final deliverance from suffering.
  It is
  said that the earth, shaken by the power of their virtue, quaked and 
  trembled.



From: MAHA KASSAPA: FATHER OF THE SANGHA by Hellmuth Hecker
Revised and enlarged translation from the German by Nyanaponika Thera      

Translated and adapted from //Wissen und Wandel// XXI, 6 (1975)
Wheel Publication No. 345
Copyright 1987, 1995 Buddhist Publication Society
                      BUDDHIST PUBLICATION SOCIETY
                      KANDY              Sri Lanka

So here we find something like "platonic love" between ascetics but in this instance much deeper, Hecker proposes the expression "karmic bond" for something which we might assume as an exotic example for a very deep arrangement of love over an already long chain of rebirthes...