That actually is addressed in the episode.Again, not the biggest problem with the episode in question, but: VOY "Fortunate Son" has an alien Delta Quadrant race claiming that they "seed" their males on different planets, from which said males find their way back to their homeworld. One of "said males" is supposedly Harry Kim, of Earth. This is taken quite seriously, and supposedly was actually considered by TPTB as a possible alternate origin for Kim.
This shouldn't have passed the giggle test! And nobody, absolutely nobody, thought to ask two key questions that would have blown the alien claim straight to hell. 1) How did you get him (I think it was embryonic "him," please don't make me look it up!) to Earth, all the way across the galaxy? 2) What reason did you have to think that Harry would even be able to get "back" to your planet in his lifetime? It's only the dumbest of luck and the most random of chance that brought Harry to that region of space in the first place.
LYRIS: Welcome. We're so glad to have you here. This is a day of celebration. Another Taresian has returned.
ELIANN: Welcome.
MALIA: You're home now, Harry.
KIM: Er, wait a minute. I still don't understand how this is possible.
JANEWAY: We'd like to ask some questions before we start celebrating.
LYRIS: Of course.
KIM: Our Doctor has found some genetic fragments in my DNA which could be part of an alien genetic structure.
LYRIS: Yes, those are part of your Taresian genes. They're becoming active. We'd be happy to provide your Doctor with some DNA samples for comparison.
KIM: What I want to know is how I could have gotten them.
LYRIS: You were conceived here, on this world. While you were still an embryo, you were placed in stasis and taken to Earth to be implanted in the womb of an Earth woman.
KIM: My mother. What did you do to her?
LYRIS: She would have been completely unaware of the procedure. After the implantation process, the child begins to incorporate some DNA from the surrogate parent. You developed physically as a human, so she would have thought you were her natural child.
KIM: But you're saying I'm not.
(The only other male there speaks. He has two adoring women hanging on his arms.)
TAYMON: We understand how you feel. Everyone here was born on other worlds. We all found our way back home like you, and discovered we're not quite who we thought we were.
RINNA: Harry, maybe you'd like to sit down.
ELIANN: Why don't you go get him something to drink?
TUVOK: All of your children are born to alien parents?
LYRIS: Yes, and when they return, each child brings an infusion of new genetic material as well as of first hand experience about another culture.
JANEWAY: But Harry wasn't trying to return here. We came to the Delta Quadrant by accident.
LYRIS: His DNA was encoded with certain instincts, including the desire to explore space. Your accident may have brought him here faster, but he always would have been driven to find his way back. You are the first to return from such a great distance.