What kind of sex are you referring to?
A kiss? Oral sex? Anal sex? Manual sex? Sex with dildos or other toys?
Is oral sex, for example, "abnormal" when two women do it, but "normal" if a man and a woman do it?
Speaking generally, it can be viewed as abnormal in the sense that only a union of male and female can produce offspring and has a practical purpose, which is favored by nature.
Heterosexual unions produce offspring = survival of the species, community (future labor force).
Homosexual unions produce no offspring, therefore no continuation of the species.
