Why do they denounce abstinence?

I am one of those atheists. The things that many religions encourage its followers to abstain from are actually natural types of behaviors, e.g. lust, or anger, and should not be abstained from. Why? Because abstinence of natural behaviors leads to frustration. If one is frustrated because they've been abstaining from their natural behaviors, it's likely it will result in indulging unhealthily. So I think that indulgence (very separate from compulsion) should be practiced instead of abstinence. Also, in terms of strictly ual abstinence, one should practice ual activity intelligently (e.g. use contraception/condoms for unwanted pregnancies and STDs).