The contention that swans mate for life is general knowledge that has been known throughout history. It is also known that there are some individual anomalies that have and do occur such as having 2 mates at the same time and/or abandoning a mate. There are several reasons that could explain this atypical behavior such as predators, some disturbance in the area that might have scared a mate away, too very young swans or one of the swans is not suitable (infertile) for mating. Nature works to ensure propagation of a species and is the reason that species members look to the healthiest and strongest members to mate and carry on the lineage.
As far as mates, if the male mate dies, the female swan will usually re-pair with another male, but this is an individual attribute. If a female mate dies, the male swan will usually not re-pair, again an individual attribute. All of this also depends on how long the mates were bonded.
If no opposite gender swans are available, young male swans raised together will bond with each other, attempt to mate and will nest. Obviously, no eggs will be produced.
If no opposite gender swans are available, female swans raised together will bond, attempt to mate and will nest. Eggs will be produced, but will obviously be infertile. We hope this information is of benefit. The Regal Swan