David had a number of concubines and multiple wives (see for example 2 Samuel 2:13). The reason his sin with Bathsheba was such a big deal was not because he cheated on his wives, for that was something he did often and would not have been considered cheating at all in his time. It was wrong because he took someone else's wife, then it was made worse by the fact he murdered someone to cover it up. Thus when considering such an enormous discrepancy between what we consider right today and what David considered right in his time, it's easy to see how a homosexual relationship would have been accepted by him without the thought that it was sinful. To make the case stronger, you could research the traditions of the time period to see if such relationships were, in fact, normal in that region. If so, I think the case for it is fairly solid. Personally, I don't care about it enough to do the research.