Well, they both know the importance of having an heir, although an earlier part of the conversation was about how it would be better for Firiel not to try and have a child immediately, to allow her time to settle in and not be too young (it is really hard to work out how the ages work with their lifespans)... but they are both thinking about and planning for the future. (btw I'm totally ignoring Aranath's birthdate given in HOME as it is two years before the date in the Appendices for Firiel and Arvedui's marriage)