In my opinion the only reason why they don't seem soo romantic in SP is becausee there are not so many scenes with these two together.. but when we see these two (where nala tells simba that kiara resembles him) they nuzzle a lot so yes! They are still in love and to answer your question I think they mate.. not "everyday" of course since they've been married for several years but quite often! And in my opinion at the end of SP they are that old!