The advantage of sexual over asexual reproduction is the genes get more mixed up. There is more genetic diversity. This means natural selection has a bigger palette to work from for evolution. Promiscuity mixes the genes even further. A bird whose eggs all have different fathers has a better chance of perpetuating its genes, rather that putting all her bets on one male. Most animals are promiscuous.
What are the advantages then of monogamy? It is one of the few defences against STDs (Sexually Transmitted Diseases). Males are more willing to put effort into raising young if they know they are his. Religion has elevated monogamy to absurdly important heights. Partners split over even one incident of straying. Humans claim to be monogamous, but in practice almost nobody is 100% monogamous. We cause ourselves so much grief with this pretence.~ Roedy (1948-02-04 age:69)