It's a famous quote, a lot of people may hear it from their father or mother when coming out to them. "You just haven't met the right girl." is a way of making the parent feel better, it gives them hope that you may still be straight. Some people may say it to themselves because they may not want to be gay and saying that they haven't met the right girl gives them hope they might not be gay after all. But is it true? Is it possible that a gay man or women could end up falling in love with and spending the rest of their life with someone of the opposite sex, happy, secure, like any other couple?
Technically, no. That statement in itself wouldnt make sense. Is it possible for a gay man or women to go on with the opposite sex, and live in content with there lifes? No. Is it possible for a bisexual individual to do such? Of course. Many drastic changes that we see in sexuality is not because the individuals sexuality actually changes, but rather it was just not fully known before. Orientation is important to connection. It is not the reason, but it is a factor that is considered in our unconscious minds.
No, I don't believe a true gay or lesbian person can fall in love with a person of the opposite sex. Women just don't do anything for me and never will. I also don't believe sexuality is as fluid as people say/want to think. This is coming from a person that identifies as pretty much 100% gay though. And there are people that don't believe that is possible either, but whatever. Hell would freeze over before I'd find a female sexually and romantically attractive, lol.