Upvote:1
Ultimately the notion of identity is an illusion. As explained extensively in the "THE PERFECTION OF WISDOM IN 8,000 LINES" sutra, there is no such thing as "sentient being". So any talk of "incarnation of same person" or really even "two different incarnations of two different people" is necessarily false, just a figure of speech.
At the same time, regardless of whether our notions are known as relative truth, karma has real effect. So any transgression is still a transgression, whether you posit an individual or not.
Sexual union in and of itself is not misconduct. It becomes misconduct when it's performed with the sole goal of satisfying one's craving for sensual pleasure. (The literal vows of lower yanas are redefined differently in the higher ones.)
What one must seek is purity of the mind, and from that purity of the speech and body action naturally follows.