Prisoner of my desire / Johanna Lindsey.

Enhanced descriptions from Syndetics:

It is a dark time for Rowena Belleme. The striking and spirited lady must produce an heir, for failure to do so will incur the dangerous wrath of a ruthless stepbrother who stands to forfeit his ill-gotten wealth. But in all of England there is only one man whom Rowena would have sire her child. His name is Warrick deChaville, a magnificent knight who possesses the strength, intelligence, and courage -- as well as the extraordinary good looks -- that Rowena would have passed on to her son. But Warrick is proud and stubborn, and he will be no man's -- or woman's -- pawn. The lady must take drastic action and so she has the noble warrior taken prisoner, intending to force him to bend to her amorous will.

Captive, chained, and seething with rage, Warrick vows he will never surrender. But the lady's seductions are powerful -- her sapphire eyes and lush, intoxicating beauty weaken his resolve, and it is the knight's own virility that betrays him. Yet all the while he is plotting a fitting and most satisfying revenge ...

For Destiny's wheel is forever spinning, and Warrick knows the time will inevitably come when the imprisoned becomes the captor, with the power to make his sensual tormentor suffer as he did in exquisite ecstasy. But neither he nor the Lady Rowena anticipates the direction the wheel will turn -- uniting intimate adversaries against a deadly common enemy ... and joining two courageous hearts in passionate, unexpected love.