Of course not. But deciding to breed is a huge step. Not just the relationship element, because if they did get home early it's something that's always going to be there but the practicality of at some point having a lot more people on the ship.
Kind of weird too to be a child conceived only so you could be trained to fly the ship home. If they were serious about this they would have to pretty much decide to go for it, pair up and start producing offspring. They couldn't leave it to random romances. A handful of children would not be enough. That's a pretty big social engineering choice for the ship to make and I'm not surprised they didn't start a few years in.
The operative word being IF, but you have to consider other factors.
Loss of crew members
How long before the oldest crew member would need replacing?
You would perhaps be talking 17 years before the first person would be able to be trained to take over from another crew.