There are serious health implications caused by extended periods in space. Muscle, bones and tissue decrease their mass and capability dramatically in just one year. Sex is possible, but a healthy newborn is highly unlikely, since zero gravity is completely unnatural for humans. Then there's a problem of vitamins, minerals, fresh food, etc. which are essential for a log healthy lifestyle. I'm just scratching the surface here.
P.S. No, I'm not fun at parties.
Yes, all those are valid concerns today. The key word being today. In the next hundreds of years who knows what kind of technologies we will develop that will solve all these issues.
Interesting. I wonder if it's not healthy for humans because we've never lived there or because we require a constant state of equilibrium to survive in?