Shiva Purana

siva.sh/siva-purana/rudra-samhita/183/26

यस्मात्सुराज्य सनसंस्थितानामंतः पुरे संगमनं विरुद्धम् ।। ततस्सहस्राणि नितंबिनीनामनंतसंख्यान्यपि दर्शयंत्यः ।। २६ ।।

sanskrit

Since it was not customary to enter the harem of kings they assumed female form of wonderful features and entered the cavern of Pārvatī for heroic activities.

english translation

yasmAtsurAjya sanasaMsthitAnAmaMtaH pure saMgamanaM viruddham || tatassahasrANi nitaMbinInAmanaMtasaMkhyAnyapi darzayaMtyaH || 26 ||

hk transliteration by Sanscript