There is no reason to assume it wasn't. While it is a bit tricky to deduce sexual preference from material culture (archaeology), we do have some literary sources. The problem with those is that they are usually representations of 'the Other', and thus possibly reveal more about the writer than about the subject being written about. Particularly with Tacitus, or later Christian writers, the goal of the writing is to teach a moral lesson about their own society, rather than objectively representing what they were writing about. You should be very aware of this bias when reading classical authors on their barbarian neighbours.

Diodorus of Sicily is one of those authors who writes about Gallia, noting that sexual contact between (warrior) men is normal and accepted. Note that Diodorus's writing is full of extreme and unnatural things, and his geography mainly a writing focused on distinguishing, and thus very interested in things that are 'different' or unusual for him. Tacitus, writing a few centuries later, writes both about almost familiar affection between warriors, but also about the death penalty for what he calls 'infame corpores'. It is possible to read this as analogous to what an Ugandan pries would call 'unnatural practices', but this is not neccessarily the only possible reading. Anyway, as a punishment he describes that the victim is to be pinned down by sticks and thus drowned in a bog. When the European bog bodies were found, those were sometimes in the past interpreted as those 'practicers of bodily shame', punished for their homosexuality. Currently, we rather see the bog bodies as ritual sacrifices, instead of as criminal punishments, for a number of well established reasons for which I would like to refer to other books, such as Wijnand van der Sanden's 'Through nature to eternity' from 1996, or Sanders 2009 Bodies in the Bog and the Archaeological Imagination, or the book 'The mysterious bog people' (don't bother with Glob's Mosefolket/The Bog People, that one's too old). The old idea of 'bog bodies as gay men' is particularly understandable if you look at the Weerdinge men (google for pictures), for example.

Other, later writers, like Procopius, are already writing from a Christian framework, and the bias in presenting the barbarians as 'others' is even more strong.

The problem with your question is that the term homosexuality (and consequentially heterosexuality) is an awkward one. To apply it back to the ancient world may be anachronistic as it relies on well defined modern categories of sexuality.

For example both Dover (Greek Homosexuality) and Foucault (The Use of Pleasure) argue that sex between men should be understood within a framework of power relations. But these works should be understood in the context of 1970's Western sexual liberation. On the other hand more recently Davidson (The Greeks and Greek Love) want's to create a history of contemporary loving homosexual relationships. However this should be understood in terms of modern struggles for the normalization of LGBT relationships (especially in regards to marriage equality)

What I am saying is that the terminology is highly coloured by current cultural trends and that any interpretation of the matter will have to take into account our own normative sexual standards.