Hi Gillian:
Your swans are doing fine and acting like swans. Nature tells the swans when it is time to mate and nest, regardless if they can produce viable young. If an opposite gendered swan is not available, then same gendered swans will pair and bond.

During mating/nesting season, same gendered swans will engage in mating and nesting behavior just as an opposite gendered pair of swans.

Males will mate with each other and build a nest. The male swans will sit on the nest, but obviously, will not produce eggs. Once the male swans get tired of sitting and their testosterone levels return to normal, they will quit sitting on the nest and go about their usual business.

Female swans will mate, produce eggs (obviously not fertile), sit on the eggs and nest until they realize that the eggs are not viable. At that time, the female swans will abandon the eggs and go about their normal business.

If cygnets are produced, then you obviously have confirmation that you have a male a female swan. So, regardless of their gender, your swans are acting normal and doing exactly what nature meant them to do. The Regal Swan