There were indications that Kaidan was supposed to be romanceable by both male and female Shepards in Mass Effect 1 but was scrapped before launch. Dialogue was recorded by Mark Meer (the VA for male Shepard) which clearly showed romance with Kaidan was possible. A modded game can open up the dialog but it is not 100% perfect; characters will call your male Shepard "Ma'am", refer to him as 'her' and in the love scene towards the end, your maleShep turns female for a few seconds. However, when a modded ME1 file save is imported to ME2 or ME3, the game registers the same-gender romance with no further glitches.
I was hoping that when the Mass Effect trilogy was announced that Kaidan might finally be available to maleSheps for ME1 but alas, it was not so.
I agree. I've read Kaidan was supposed to be bisexual from the start but they edited it out shortly before launch, the same with Ashley for femSheps, I believe. I've played the modded romance too, and there's still quite a bit of SGR content left in the first game.
BUT it was quite possible to play through ME1 and ME2 without those options and still find the romance between Shep and Kaidan to be convincing. Basically, the whole thing had the "You know, Shep, I've been thinking... " vibe and Kaidan revealing that he'd never been serious about other people in the past. He didn't explicitly mention that he'd been bisexual (or pansexual) all along, but it was what you gathered from his conversations with Shep.
I could see Corso in that same vein. You know, country boy, hasn't experienced much and now there's this whole new world to discover..... but we'll see.