Wait what? He wasn't a m/m LI in ME1? O_o Never went that route (he's not really my type) but took it as a given from the body language between him and mShep 
No, he wasn't available in ME1 for men. There is some lines buried in the game's data for a m/m Shep from him, as well as lines from Ash towards a femShep, but that's it.
You know, the body language was identical to femShep in ME1. Same nervous look, same brow wipe, all of that. I think he's just hot for Shepard, no matter what.
With a Sheploo, he's always probably been attracted to him, just Kaiden needed some time to wrap his head own his own feelings. Since I know bisexual/pansexual folks can come out at any age at anytime, it would fit for me that he finally got his head around his sexuality just in time for the possible End of the Galaxy.