Trick or Treat? by Ray Connolly

The blurb on the back:

Triple Treat
Kathy, a luscious Californian blonde, arrives in Paris looking for the gay life. There, she soon falls prey to the advances of the enigmatic Ille, an exotic Eastern beauty skilled in the sensual arts. A passionate romance develops. Together they share everything. But it is not long before they discover that the intensity of the relationship demands more than either girl alone can offer.
They want desperately to be a family.
They want a baby.
So, ironically they need a man. A man who's willing to play their sexual-games and become their unsuspecting stud...
Trick or Treat is a frank, outspoken novel about erotic game-playing and romantic taboos, but above all, it is a story of sinister sexual manipulation.