Sex on the Odd Side by Lois Macdonald

'Helen could be made happy by Lya, who had never touched a woman until Helen had taught her how. After that, Lya couldn't get enough. But Helen could also find sexual fulfillment with John, who was unaware of the other lover--the lesbian--in Helen's life.'

'the eternal triangle with a corrupt twist- a man, his wife, and the woman she loved!'