Someone please correct me, if I'm wrong.
IIRC, two months after the Prince bonds, he is supposed to take the Throne. Except this particular bonding is male-to-male. Something not seen before, and certainly no chance of offspring. That would end the line of succession.
Lachlan, the King, is attempting to train Rory just enough to send him to the front where Lachlan hopes he will be killed, thus breaking the bond. Lachlan then hopes that his son will procreate with the woman who is "engaged" to Kieran, and save the bloodline.
However, Rory went into battle with his Prince and helped not just to save the Prince's life but helped greatly to defeat their enemy. Rory's magic is wild and has tremendous power. Lachlan will know of this and see his plans falling apart.
At this point, Lachlan will not give up the Throne until the magical bond between Rory and Kieran has broken and Keiran can have sex with someone other than Rory. i.e. the supposed woman he is publically engaged with, to prevent the populace from knowing about the same-sex bond. Incidentally, this saves the bloodline.