No, we cannot rule out the possibility of life on other planets, but we know from God's written record that we've got enough on worrying about ourselves and getting ourselves right with God! I think we can trust God to deal with any problems other life-forms on other planets might have!
Now, the real point of your question - in what sense is Jesus the only-begotten son of God? You might be surprised to learn that Christians state that the Father, the Son and the Holy Spirit who co-existed from eternity, managed to create the universe and all life on it without any recourse to sex. They let their creatures reproduce asexually, or sexually. There is no sex in heaven. God is neither male nor female. When the Bible speaks of the Father and the Son, that is to express the relationship between those two Persons of the Godhead.
When it came time for the eternal Son of God to take on human form and become the man, Jesus of Nazareth, the Holy Spirit changed His life into the womb of the virgin Mary. No sex involved. Jesus the Son is uniquely the Son by His very Being. Angels are called sons of God, by virtue of the Father, the Son and the Holy Spirit creating them in heaven. Humans are called sons of God by adoption into God's family. You must stop listening to the Mormons who DO teach sexual reproduction by God with his wives. This is blasphemy. Mormons are not Christians for that very reason. I hope that helps clear up any confusion.