Question

how would you explain that B2 molecule is not magnetic

Answer 1

For the same reason O2OX2 is paramagnetic: degeneracy.
The filled orbitals are σ(1s)2σ(1s)2, σ∗(1s)2σ∗(1s)2, σ(2s)2σ(2s)2, σ∗(2s)2σ∗(2s)2. The half-filled orbitals are π(2px)1π(2px)1 and π(2py)1π(2py)1.

But you're right to think that this is unexpected: why are the π(2p)π(2p) orbitals lower than the σ(2p)σ(2p)? That's the reverse of what we are told to expect! I've never encountered a rationalization that satisfied me.