The story of David and Jonathan is one of those most frequently quoted in any discussion of biblical same sex relationships. As with the stories of Ruth & Naomi, or of Jesus and John (the "beloved disciple"), it is similarly bedeviled by discussion over the degree of physical intimacy involved (was there or wasn't there?), and the impossibility of knowing for certain.
Personally, I see these questions as something of a distraction, just as I do with the other cases. Gay men are frequently accused of being "obsessed" with genital sex. If we only accept as "gay" those men for whom we know there was this genital activity, we are simply reinforcing the stereotype. I prefer simply to recognize that there was clearly a deeply intimate emotional relationship here, and to ignore the degree of physical expression. (Chris Glaser has pointed out that whatever the nature of the relationships, the stories of David & Jonathan, and of Ruth and Naomi, are the two longest love stories told in the Bible - longer than any obviously heterosexual love stories. Marriage in Biblical times was not about love. See "Coming Out as Sacrament")
However, for those who are determined to dig deeper, there is a reference by John McNeill (in Sex as God Intended) which is worth thinking about.