Bond Dissociation Energy:
F-F: 156.9 kJ/mol
Cl-Cl: 242.58 kJ/mol
Br-Br: 193.87 kJ/mol
I-I: 152.55 kJ/mol
Bond dissociation energy of F2 is lower than Cl2 and Br2 (though not I2, due to the low electronegativity of iodine).
Fluorine is a highly electronegative element.
From the picture we can see that the fluorine molecule is a pretty small one as compared to its group members. Atomic number is 9, 1s(2)2s(2)2p(5), seven electrons on its outermost shell and a very small atomic radius.
Two fluorine atoms must come together to share a pair, so that the octet is completed.
But these many electrons concentrated on two small nuclei and separated over comparatively a very small distance repel each other.
Hence the F-F bond is weakened.
[Note: This is similar to the low BDE of peroxide bond (-O-O-), where the lone pairs of oxygen atoms repel each other, eg. H2O2. Its BDE is 142 kJ/mol]