"But whoever seeks beyond that, then those are the transgressors," meaning: the fornicators; they transgress from the lawful to the unlawful.
﴿فَمَنِ ابْتَغَى وَرَاءَ ذَلِكَ فَأُولَئِكَ هم العادون﴾ يَعْنِي: الزُّنَاةَ؛ يَتَعَدَّوْنَ الْحَلالَ إِلَى الْحَرَام