They started dating after their loved ones' coffins were lowered into the waiting ground. She needed somebody who wasn't the usual somebody, he needed help to keep his world from shattering. They had common interests and people in common, and if it wasn't love, it was, at the moment, smart.
Besides, it was a great cover for their shared hobby. Made travels and downtime easier to explain.
Dinah and Helena were dead and sense and reason seemed hard to come by. But he had all the questions and she had all the answers.
Ra's al Ghul was going to die.