The Saiya-Jin’s ultimate goal is to be the strongest one in the universe. This is why they all train so hard. This is also why they selectively choose their breeding partners. Even though a Saiya-Jin might be mated, he will still impregnate another who he will be able to create a strong offspring with.
Saiya-Jins know that, by mixing blood, incredibly strong warriors can be bred. This is why they often pick out the most talented women from the planets they conquer and use them to breed new Saiya-Jin. These Saiya-Jin will also take on traits of their mother and will therefore have different looks about them.