In the European PC version of ME1, both Kaiden and Ashley were SGR capable. For some reason here in the US, ONLY Liara was kept as an SGR. At least, that's what was said on the youtube videos I watched. I would link them, but I would probably get warned or infracted for it. :/
DISCLAIMER: It MIGHT have been a hack, since I haven't been able to get my hands on the EU ME, yet.
It was a hack, basically making Kaidan (and Ashley in ME1) available as SGR option through changing your Shepard's gender. That was only possible of course because Ash and Kaidan were supposed to be SGR options from the start but were edited out at the last moment. Only thing was, Kaidan did refer to you as a female from time to time, and the last romance scene had Shepard change to femShep for the most crucial part. There was a nice kissing scene left though. You could carry over both romances to ME2, triggering short cut scenes for both options, yet only Kaidan remained available as an official SGR option in ME3. The hack was working for EU and US versions respectively. I played through ME1-ME3 with Kaidan as SGR option throughout.