I did this from my middle teens to my late 30's. About that time I started losing interest in sex and women in general so eventually I stopped.
None of my imaginary girfriends were based on real women. They were all figments of my imagination. However, they all tended to be part of a minority in society. For example, one IGF was an android whilst another was a mutant whose DNA had been infused with that of a cat (not sure why a cat - maybe because I wanted some p**** LOL). Funnily enough, one of my IGF's was an Autistic girl named Sally who functioned just slighly worse than I do. My fantasies with my IGF's all had complete story lines including how we met, when we started dating how we lived together and how we overcame various problems that threatened to tear us apart. I look back on these fantasies as some of the best memories of my life. In a way, I think having been satisfied with these fantasies for many years, I would be disappointed with any "normal" woman I might meet.
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I am highly in tune with my perceptions. It's reality that I haven't got a clue about.