It's not terribly abnormal. It's certainly considered less abnormal than other sexual behaviors, such as S&M. It might not be what the majority of people like to do, but that's just a question of preference. As for it being destructive, any sex act among consenting adults is not destructive, in of itself. How would you describe destructive?
Lesbianism may be less destructive individually (compared to MSM) but it's certainly destructive at the societal level especially in the cases of persons who actively promote the behavior.
How does a lesbian sex act harm the rest of society?
Families aren't a societal luxury, they are the bedrock of humanity.
Homosexuals make about 3% of the population. How does the existence of such a small % of people practicing a certain type of sexual behavior harm families?