So I've just watched a video that listed "5 gay armies in history". The first three I already knew about: Samurais, Greeks and The French Foreign Legion.
The next two, however, I've never heard of- Islamic warriors(Mamelukes, Janissaries etc.) and Celts. Was there a great number of homosexual acts going in these groups? I know they wouldn't have considered themselves gay, but did they have a lot of gay sex?
I don't know enough to comment in this particular aspect, but keep in mind that both these 'Islamic' groups were basically ethnic mercenaries. The Mamluks were of Caucasian/Circassian origins and were initially hand selected to be a personal 'owned army' of an Abbasid Caliph. The word Mamluk means 'owned'. The Janissaries were likewise a hand selected elite Army corps of exclusively Christians. This might or might not be relevant to your question, but the detail is important to distinguish them from, say, the first Mujahadeen or Armies of the Rashidun Caliphate which were properly 'Islamic' Armies.
While this is not exactly an example of "homosexuality" as much as "rape-as-an-act -of-dominance", rape of male prisoners and captives was prevalent in medieval Turkey.
On of the victims of such a rape was Vlad III Drăculea of the Bassarabi dynasty, who was a turkish captive as a boy.