Well it would have perhaps been more realistic to have the crew pair up and start creating the next generation of crew who would man the ship. Just add it to the list of things that VOY gets critisim for.
From an in-universe point of view, the crew didn't know how long it was going to take them to get home, so a logical course of action would be to start having babies in case it did take the 70 years.
From a production point of view, they knew the ship would get home in seven years.