With the fact that Dumbledore's homosexuality is finally known, there could be bit of hints in the series that may have implied it such as these:

1) No mention of his love interest

2) "flamboyantly cut, plum velvet suit" in a flashback scene in HBP

3) His reaction when Harry told him that he said to Scrimgeor that he is Dumbledore's man " through and through "

If there's anymore hints you can find please post them.