1. Sexual strategies of men and women are inherently competitive. Monogamy is the best known and tested way to find a compromise.
2. Encouraging monogamy reduces male violence, jealousy and STI rates. It also boosts the net romantic and sexual satisfaction of the entire population across many life stages, creating a stable environment for raising children and acknowledging that physical intimacy, romantic love, engagement and life arrangements are deeply interconnected.
3. Modern polyamory is basically the feminist disguise for open polyandry, increasing women's sexual access to the most attractive men and turning all the nice guys into stereotypical beta providers who get lots of drama and little to no sex or emotional involvement (the latter one should be important for asexuals as well).