Taboo by Barbara Brooks

'It began as an act of sudden impulse but became a habit neither of them could break'.

From the back:

An odd affair:

Grace... the professional party-girl, feeding her hungers for thrills and sensuality without caring what effect her unrestrained sexuality might have on the young girls she dazzled with sophistication and captivated with dark passion.

Kim... the pretty career-girl, swept up in the newness of a forbidden love, forced to sacrifice all pride and self-respect in order to satisfy and keep the woman who had awakened so terrible a need.