For example, in ethene the sigma bonds are stronger overall than in ethane.
Ethene (pKa = 44) C=C 108 kcal mol-1 (+18) C-H 111 kcal mol-1 (+10) Ethane (pKa = 50) C-C 90 kcal mol-1 C-H 101 kcal mol-1
This is because the degree of s character is higher in the sp2 hybridized bonds, giving more compact and stronger bonds and thus reducing acidity.
This greater s character also makes the carbon more electron withdrawing, polarizing the bond and making the alkenyl hydrogen more acidic.
It seems counter intuitive that the stronger bonded ethene H is more acidic than the weaker bonded ethane H. I thought the increased bond strength would outweigh the increased bond polarity. It is like saying: "this bond is stronger, but it is also easier to break."