is homosexuality genetic or is it learned behavior?

This is partly a myth. Not many soldiers had sex with each other. There were prostitutes. Sometimes they might even go with a male prostitute because they were cheaper (half the price), but probably not often. (Maybe the very cheap and very horny ones found a "buddie" to hook up with, to try to save money and find an outlet for their carnal releases)
Some soldiers refrained from having sex with other persons.

 

There are a few theories about how it could serve some role.

I'm not sure any of those theories are extremely convincing, but I could be wrong.

In ancient times the pairing usually involved two unequal individuals, one of them older with more power and money.

So one of these theories goes that maybe if a younger bisexual individual got pushed out of the nest, there might be another nest to go to and they could be more likely to survive and perpetuate their genes.

There are all sorts of examples in nature of animals and plants with more than one sex so this phenomena could be analogous to that.