A nation is a body of people. A body is made up of many organs, and those organs are made of cells. In order to perpetuate whatever organ the cell comprises, it must be capable of replication. Therefore the fundamental unit of a nation is the heterosexual family of 1 male and 1 female, this is also scientificly proven to be the best system for child rearing, if your ancestors doing this for thousands of years isnt good enough for you.
Homosexuals of all varieties and line-enders such as trooners and pooners express behaviors aberrant to the perpetuation to the body by not only not self replicating but encouraging other cells to also not replicate. They devote massive resources to their own behaviors that again run counter to the basic purpose of the body. In nature we would call this behavior "cancer."
People with heterosexual but non monogamous paraphilias while can sometimes still hold potential for that basic purpose do so in a manner that negatively impacts the child/children in ways either slight or significant that could almost always be avoided by adhering to the proper model described above.