Fornication in terms of peace treaty how sinful?

Bonobo monkeys do it extensively. They make symbolic sex as an act of maintaining peace and resolving agression. How is this with Medieval kings that had (to have) sex with offered women from other tribes or neighboring countries. If he was forced to marry princess(es) of other kingdoms. Is this fornication? Lets add the factor of lust of flesh. Does it make a difference if he enjoys doing it or if he just does it as a necessity? (Having many women) How sinful is that, please differentiate, if it is sinful at all or what are the options. What if this act of 'self-sacrifice' is a measure of preventing war, which would cost many lives? To mention King Solomon as an example, he had some 1000 women, but the problem was, that they turned his heart away from god and he worshipped other gods and fell into false idolatry. What if this 'heart-flipping' was not the case? How sinful is that?