Remilia is from an age where not every child was assured of reaching adulthood. Hell, Flandre almost didn't. Of course, it could be argued that she still hasn't yet, so six of one. But even back in the 1700s, when Remilia lost track of the outside world, child mortality was appalingly high.

It could also be that Remilia is thinking of the truly long term, given she's taken an immortal as a fiancee; who's to say that 20 or 50 or 100 years from now, her husband might not long to hear the pitter-patter of little feet in the house once again? He and Kei didn't contemplate them at all for almost three centuries after getting married.

Anyways, if Gryph really wants children with Remilia, I'm sure 25th century technology is up to the task. Vampires and Detians are both "human+", and matching up the genes shouldn't be that difficult, especially since they have Flandre as an example of a vampire with Detian blood.