What? Those manly Vikings, spending all that time together dressed in leather...

Well, probably not. The problem is that there is almost no clear written information from the Viking Age (600-1000ish AD) in the Vikings own words. What we do have is information from the Icelandic sagas, written several hundred years+ later, and after being 'polluted' by Christianity. This article here from Fordham University suggests a number of things based on linguistic evidence and analysis of stories. http://www.fordham.edu/halsall/pwh/gayvik.asp

That penetrating another man is not considered 'gay', but receiving penetration is. That being penetrated was an innately passive and 'weak' act to receive, thus any man doing so was a lesser creature. That a mans duty was to procreate and your preferences were unimportant. That being masculine and a father/senior male figure was all important, that being a receptive or femine figure was not. Rape of a male was fine and was something you might do without question to a defeated foe.

As such it was probably normal to have a male that you might fuck, but it wouldnt be considered a co-equal relationship and the reciever was very much the 'bitch'. As such it mirrors the way some prison sex-relations work (reputedly). This was more a power-relationship than it is a sex-relationship. But giving it to someone was in and of itself not prohibited. Unless perhaps you seemed to prefer it.

Again, this information is extrapolated from writings written a fair bit latter and may reflect their own society moreso than the true pre-Christian Vikings. Who may have been going at it like Rabbits for all we really know, however the Icelandic evidence does 'gel' with what we think we know about similar societies.

/u/Thecna2 hints at some of the difficulties of answering this question: it's very hard to find evidence for homosexual relationships from the kinds of evidence that survive from the Viking Age. The written sources, as already stated, were written several hundred years after the 'Viking Age,' and can't be trusted to reliably preserve cultural attitudes from the 9th or 10th centuries.

Archaeological evidence is also not very good at revealing homosexuality. There are a few graves where two men are buried together, and these are sometimes interpreted as the graves of gay lovers. But this interpretation is very subjective; we have no evidence to suggest that two straight men couldn't share a grave in 10th century Denmark.

There's also the more difficult conceptual problem of terms like 'gay' and 'straight,' which are cultural categories that grew out of 19th and 20th century social contexts. Same sex attraction is a human thing, but different cultures express that attraction in different ways, and (as /u/Thecna2 shows) these expressions don't always manifest as the sexual orientations we're familiar with today. Lacking good textual sources for the Viking Age (ie, a 10th century text which describes Vikings' ways of expressing same sex attraction), and lacking the ability to tell who someone was sleeping with from archaeological remains, we're left with more questions than answers.