I don't know of any mention of this in Siri Guru Granth Sahib.
Sikh Dharma is a set of guidelines for humans. If we follow it, we will be exalted and escape from the cycle of reincarnation. It's not a set a rules to be used to judge and condemn people. We accept all, because God is in all. The problem with homosexuality is that it leads to imbalance in the phsyce of the person. The male and female have opposite polarity, so when they merge it creates a balanced energy. In a homosexual relationship, the phsyce becomes too masucline or feminine so it becomes very difficult to maintain balance in the other areas of life. That can lead to self limiting, or destructive behaviours. It is really a personal matter, part of the Karma people are born with. We can only provide support and encouragement to all, regardless of their personal proclivities.