Thank you all for providing contributions in this thread and sharing your thoughts. I feel like revisiting for myself and providing some comments.
There are three possible approaches to seeing or seeking wisdom through Sri Guru Granth Sahib Ji.
1. We learn from those who may have understood and do little study ourselves. This perhaps is the dominant style in Sikhism out of sheer convenience and the easiest to passively participate in. We love to have things provided to us on a platter. Is it all wrong? Of course not but we need to be careful and watchful as to understand the difference as to person we believe understands versus those who pretend they understand and they want you to close your eyes and follow them.
2. We seek answers out of Sri Guru Granth Sahib Ji versus trying to understand Sri Guru Granth Sahib Ji. This is again a fast and convenient approach where we may believe that Sri Guru Granth Sahib Ji is a book of answers versus a teacher. A book of answers is limited to the number of questions answered and is rarely a good teacher. For me Sri Guru Granth Sahib Ji is way beyond a book of answers but a supreme teacher. Some subjects if so thought by Guru ji's and others may have been addressed head on but not all is possibly addresses in questions and answers format. I believe the subject of this thread falls into that category as I did search a bit to find specific questions and answers dialog but was not successful though I stand corrected.
3. The essence of Sri Guru Granth Sahib Ji is as a teacher for me. What a great teacher allows a student (Sikh) to do is determine answers through understanding. This is the most difficult of approaches but one that liberates us through understanding where we do not follow shortcuts and have lesser chance to fall prey to misguidance.
So from the perspective of this thread I believe we may say that the following fall into Category 1 where we believe in wisdom of those who brought us the Sikh Reht Maryada or those of our family and others that we chose to follow,
I am guided by Sikh Reht Maryada in these matters and marriage to be as monogamy, heterosexual and to be between Sikhs only.
I am socially or parentally following clues or direction.
Some have tried to quote from Sri Guru Granth Sahib Ji but I believe it has not been convincing.
The question of free will is a paramount principle in Sikhism. However free will is not unencumbered. It is associated with knowledge, understanding, truths, and so on with Sri Guru Granth Sahib Ji as a marvelous teacher enabling us so. In true spirit of Sikhism the following three are expressions of the free will and understanding tenets.
It is of my free will and no one else's business.
I wish Sikhism evolves towards free will in these matters.
To each their own, I accept other people's decisions about their own lives and living.
Just some thoughts as we converse and share.
Sat Sri Akal.