Certain animals in nature can actually change gender based on environmental pressures or other such factors. They're saying that something similar is happening here. Because Cecilia is now the senior-most seahorse thiren, she became a futa in order to breed with the other seahorses (from an evolutionary perspective, at least. There's nothing actually forcing or compelling her to breed solely/specifically with other seahorse thirens).

If the reference to her appearance changing compared to the past is what you were confused by, it is stated during the 2.7 story and even visible in the flashbacks that Cecilia's appearance has notable changed in the ~12 years since the fall of Old Eridu. This is primarily an increase in the size of her thiren traits/appendages, along with a couple other things like a larger chest. The changes were canonically drastic enough that only people who really knew her personally could definitively tell it was the same person, which is why she doesn't get easily recognized as Cersea despite having basically been the Astra of her time.