A New Kind of Love by Anne Herbert

The blurb on the back:

Lillian belonged to the oddest sexual twilight - a lesbian in love with a man.
Lillian needed John, wanted John - but she wanted, she had to have, her lesbian lovers more. Like the young girls she found in a 'gay' bay, and took home to initiate into the ecstasy of homosexual love. Or the older woman with the bizarre variations on the usual lesbian love-making.
When Lillian first met Cathy, it was just a casual pickup, to last a single lust-filled night. But Cathy was different ...