So far as I know, there have been no studies on human reproduction in space. I think the largest problem would be the simple mechanics of the activity in a weightless state.

As for the cellular issues, I don't think biologists have uncovered any significant mechanical problems with cellular reproduction in other species of simple orgainsis (ants, ameoba etc) that have been examined in space far. Cells, after all, seem to function in a near-weightness environment provided by aqueous solutions. One issue that is probably a severe liability is the radiation environment of space, even inside a spacecraft. These factors are known to cause chromosomal damage from even short-term exposure.

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