I've found that norse mythology had a concept of unmanly sexuality which resembles roman attitudes toward sex, which could conceivable lead to homophobic attitudes although apparently the ancient romans didn't mind homosexuality as long as you weren't the submissive partner so maybe that's how it was with the norse as well. That said, I haven't found any other evidence even suggestive of homophobia that predates the abrahamic doctrine. Do you know of any?
Yes, the very Norse concept you mentioned, known as ergi, could be considered homophobic.
The Scandinavian peoples believed in a concept known as drengskapr, which was the difference between a man "maðr" and a real man "drengr". It was a very important word to the Norse people's idea of honour, the word itself meaning things like manliness, high mindedness and courage but the interesting thing is that this concept of drengskapr wasn't restricted to your genitalia and instead valued certain virtues over sex. However that's not to say it was entirely equal.
Take for example the opposite idea of drengskapr, Ergi, which meant unmanliness first and foremost but also had different concepts attached to it much like drengskapr. To be Ergi is to be entirely the opposite of all manly virtues, things such as showing cowardice, being selfish, dealing with magic or most importantly playing the part of a woman which most commonly referred to being penetrated or the submissive partner in sex.
Eldar Heide said this on the topic of Ergi
“for a man to be penetrated by another >man was the ultimate unmanliness, >because that symbolically turned him into >a woman. Injury in the buttocks had the >same effect.”
When someone was accused of not meeting the cultural standards for being drengskapr it was assumed they equally lacked in all other drengr virtues. An Ergi was also subject to níð, which are illegal formalized insults in Norse law. These insults included comparing him to female animals (mare, bitch, etc), saying he had given birth, performed sorcery or even played the role of a woman every ninth night. To understand the gravity of being subject to níð it's good to understand that the níð itself was a crime comparable to rape or murder and punishable by full outlawry, which in this time period meant to literally live outside the law without its protection.
Does all this mean that homophobia was a commonly held belief among the followers of the Norse faith? I'm honestly not sure but I'll leave you with an example of drengskapr/ergi in Gisli's Saga.
A farmer named Thorbjorn had a gorgeous, strong-willed daughter named Thordis. When she reached the age of marriage, a suitor by the name of Skeggi the Dueller – a rough brawler, as his nickname implied – came to Thorbjorn’s farmstead and asked for Thordis’s hand in marriage. Thorbjorn refused, and offered a half-hearted excuse. But everyone knew the real reason why Thorbjorn had turned down Skeggi’s request: Thordis was already seeing another man by the name of Kolbjorn, a well-to-do local farmer. So the Dueller, angry and humiliated after being rebuffed, stormed off to Kolbjorn’s house and challenged him to a duel for the hand of Thordis. Kolbjorn was terrified, but agreed, noting that no one would think him worthy of Thordis if he declined. On the night before the duel was to take place, however, Gisli, Thordis’s brother, went to Kolbjorn and asked him if he was prepared. He found Kolbjorn panic-stricken and questioning whether he would actually go through with the duel. During their conversation, Kolbjorn asked the stronger, bolder Gisli to fight in his place. Gisli harshly ridiculed Kolbjorn, calling him the most despicable lowlife in the world, and saying – correctly – that Kolbjorn would bear a heavy shame for the rest of his life. But, in the end, he agreed to take Kolbjorn’s place. The next day, Skeggi and his men arrived at the appointed location and waited for Kolbjorn. A long time passed, and neither Kolbjorn nor Gisli showed up. So Skeggi said to his carpenter, Ref, “Carve two life-sized, wooden statues of Kolbjorn and Gisli. Set them up with one standing behind the other, penetrating him. And may this monument of scorn (níð) remain here forever to mock them!” At that very moment, Gisli appeared from the woods. “Give your farmhands something better to do,” he sneered at Skeggi. Waving his axe in the air, he growled, “Here is a real man, one who will fight you!” The duel raged. Both men fought fiercely and skillfully. But eventually, in one tremendous sweep of his axe, Gisli sliced through Skeggi’s shield and into his leg, severing it at the knee. Despite his unbelievable pain, Skeggi had the presence of mind to know that he had seconds to save his life. So he immediately offered to pay Gisli a great sum of money to end the duel then and there and not kill him. Gisli accepted. From that day forward, Gisli’s victory increased his reputation considerably, while Skeggi had to walk with a wooden leg – presumably this was the use to which he ended up putting Ref, his carpenter.
There are zoroastrian texts that strongly penalizes homosexuality (or rather, anal intercourse) like with most zoroastrian texts it's hard to get a definitive dating though. (Encyclopedia Iranica has an article): http://www.iranicaonline.org/articles/homosexuality-i)