Biologically speaking i would suggiest its abnormal. Sexuality is for the purpose of procreation. Homosexuality precludes that. From a biological pov it is therefore abnormal. Its not a useful genetic quality as it precludes the passing of genes.
That said, in creatures on the level of humans, sexuality is more than simple procreation. Its common and therefore not "abnormal" or a problem. Its a matter of base line doscussion i suppose and what specific angle you are addressing.