Why is ammonia less acidic than terminal alkynes?
It is the alkyne that is not in line. 
In the alkynes, the carbon is sp hybridized and this gives it much more s character. 
Because of this, the electrons are closer to the carbon nucleus and the $\ce{C-H}$ bond is slightly polarized. 
The carbon has some &delta; - and the hydrogen &delta; +. 

Therefore the terminal alkynes are more acidic and have a lower $\mathrm{p}K_\mathrm{a}$ than ammonia.