"Big1- So let me ask you this, is it possible that the sexual side effects of propecia could appear a year, or more, into taking the medication?"
They may continue to exist, I imagine, but they would become prevalent much sooner. No, it is not likely that you would have to wait a year before you notice "sexual side effects".
"Maybe the problems I'm experiencing are stemming from something else ?"
They probably are. Don't worry about your girlfriend; if it really becomes a problem just use a donger on her.
Keep your hair. That's what is important because no transplant can give it back to you; several transplants can make a HUGE difference but they will never be as good as nature.
"BTW, is it true that it takes about 1 year of continious useage of minoxidil for it to start seriously working?"
I don't use Minoxidil. I take one pill befre I go to bed and that's it. Combine that with surgery (with an excellent doctor) and excellent results are imminent.