
For me personally, so much character development occurs through the complex interactions between my character and those written by other people. To skip all the experiences and interactions that may lead to an organic connection between them is to deny my character what I feel to be genuine development and progression within the world. A large part of the creative appeal of roleplay for me, as opposed to writing on my own, is to watch my character develop through unpredictable situations and engagements with others.
I feel that a prior arranged romantic roleplay would rob that creative process of its organic value and in turn, result in a character that feels forced or unnatural. It would affect my immersion.
While I certainly don't judge others for pre-arranging romances, or seeking out that kind of writing specifically; I feel that the relationships my character forms with others is part of the greater whole of who she is. Romance is never the central theme to the story of any of my characters, only a supporting plot that may or may not enhance her story and development.
From my experiences, I find that the more forceful someone appears OOCly for their character to be involved with my own, the less receptive I become to the idea even if it may have been a natural step my character could have taken. I distinctly recall, my character becoming involved with another (a different game) and when I in a friendly way said I 'missed them' meaning the writing during a very busy period of my life the person's response was, "You don't miss them. You miss me."
Needless to say, I did not continue to roleplay with that person because they had begun to blur the lines and since have grown cautious of roleplay with too heavy a romantic component.