There is a difference between 'mates' and 'true mates'.
Mates are two people, or rather two Saiya-jins, who share similar personalities and interests that decide to enter a series of rites that will determine the nature of their relationship. Sometimes the relationship can be sexual. Other times it can be friendship that evolves into sexual activities and returns to friendship again. Occasionally, mates will stay together and produce offspring with one another. However, it's also not uncommon to see a mated pair decide to have sexual intercourse with someone else while still remaining together. Depending on the mated pair, the male or female can have children with someone else without angering their partner. Sometimes, the child can be brought into the family unit if the [here the word is scratched out] does not wish to be apart of the child's life.
True mates are two Saiyajins who are compatible in almost every way. There have been records of Saiya-jins from the Elite class having Mid class true mates and an even more shocking instance where a member of the Royal family had a Low class true mate. The way of telling whether one is True Mated or not is based off of a critical moment in the person(s) life, and/or the tail. Normally, true mates are born close in age to each other though for there to be large gaps isn't uncommon either (see Saiyan Lifespan for details).
It is speculated that true mates come in contact semi regularly or constantly with one another over the course of their lives and don't realize what they are to each other until they are on the verge of death or their world view changes irrevocably such as the birth of a child, going into battle together and their battle energies rise/spike in tandem (see Battle Energies for details). If none of this happens, though, the fail safe is the tail. While grabbing a Saiyajin by their tail is painful, crippling, even, it's also very rude and frowned up; caressing the tail while in the midst of intercourse is pleasurable but isn't something to do with a casual sex partner. Post coitus, it is normal for male Saiyan's tail to curl and flex before wrapping around the waist once more, however, if the tail wraps around the female (or male)'s own tail and remains locked in that position for more than a five minutes, they will begin the process of Bonding (see the Various Degrees of Bonding for details). DO NOT TRY AND SEPARATE.
True Mates are normally stronger, faster and better, together. If they are Bonded, it is rumored that they can achieve the legendary Super Saiyajin status however this is considered a myth. When one dies, the other will know and enter a feral stage similar to the first time one transitions into a Great Ape; a mercy killing is often required.