Both trauma and biology can (on their own) lead to same-sex attraction. As to the biological mechanisms:

The four major types of organizational permutations can yield the obvious forms of cross-sexual gender identities: the presense of malelike brains in female bodies and of female brains in malelike bodies. [This] arises from a simple biological fact. The signals that trigger babies' brains and bodies to take the various possible gender and sex paths are separate.

Affective Neuroscience: The Foundations of Human and Animal Emotions by Jaak Panksepp, 1998, Page 232 (of 466)

Jaak Panksepp was an interesting psychologist and neuroscientist:

https://grokipedia.com/page/Jaak_Panksepp on Dr. Panksepp:

Born in Tartu, Estonia, he immigrated to the United States as an infant in 1944 and earned a BS in psychology from the University of Pittsburgh in 1965, followed by an MS in 1967 and a PhD in psychobiology/neuroscience from the University of Massachusetts Amherst in 1969, where his dissertation explored the neural basis of aggression.[1][2] Throughout his career, spanning faculty positions at Bowling Green State University from 1972 to 1998 and later at Washington State University from 2006 onward—where he held the Bernice Gilman Baily and Joseph Baily Endowed Chair in Animal Well-Being Science—Panksepp authored over 400 publications, including seminal books like Affective Neuroscience: The Foundations of Human and Animal Emotions (1998), which has garnered more than 7,000 citations, and The Archaeology of Mind: Neuroevolutionary Origins of Human Emotions (2012, co-authored with Lucy Biven).[3][2]

Panksepp's most enduring contribution was the identification of seven primary emotional command systems in the mammalian brain—SEEKING, CARE, LUST, PLAY, FEAR, RAGE, and SADNESS (or PANIC/GRIEF)—which he posited as evolutionarily conserved circuits driving core affective states and behaviors, with the SEEKING system serving as a central motivator for exploration and reward.[1] His research emphasized the study of emotions in animals, including innovative work on ultrasonic vocalizations in rats to infer positive affective states like joy during play, challenging prevailing views that dismissed animal emotions as anthropomorphic projections unfit for scientific inquiry.[2] This framework not only advanced basic neuroscience but also had profound implications for clinical fields, linking dysregulation of these systems to psychiatric disorders such as depression, anxiety, and addiction, and informing therapeutic approaches in biological psychiatry.[3]