It doesn't make it impossible, though.
It does make it more inconvenient, though.
Surface area pressure vs. point pressure of a curved surface. Curved surface would require a hardpoint at the lowest point to bear the stress of the craft's weight. A flat area, on the other hand, divides the contact force to much larger area.
On the other hand, modeling in some sort of landing gear structure to the bottom of the bays could be done anyway.
For the record I'll say I would prefer more angular shape of the secondary bays and don't care so much if the guns are triangular, round or star-shaped.