His father had mated with his mother - there might have been love between them (who knows?) - but Sesshomaru knows that his half-brother's mother was his father's true love. Of course that stings.

Where demons are concerned, Sesshomaru is still in his teen years, so he has plenty of time to reconcile his feelings on mating.

"I have never felt that for anyone." he has loved and protected - Rin being his main focus - but he doesn't view her in a romantic light.