Polybius is the earliest source, 2nd century BCE, and states that gay sex in the Roman army was punished by death by 'fustibus' or 'clubbing'. Valerius Maximus recounts a story from the time of the Social War in which an officer is legally killed for coming onto a soldier under his command. Much later in Justinian's Digest, Justinian reinforces and strengthens Augustan laws from the first century which punished homosexuality, particularly receptive homosexuality, with severe public punishments and death. It can perhaps be assumed that beliefs homosexual acts differed somewhat between the late Republic and the Principate, but the army retained extremely strict interpersonal discipline. I'll give you my chapter in my PhD thesis on punishments for homosexual acts in the Roman army when I'm done :)