Human biology dictates that two sexes are required for the production of a baby. Babies are necessary for the continuation of the species.
Darwin and others have pointed out that to a great extent, animals seek to extend their genetic material into the future through reproduction. Much of what animals do is a competition to see that their DNA makes it to the next round at the expense of others'.
Some primate males practice infanticide against other males' babies so that they may mate earlier and produce more offspring.
Milne-Edwards' sifaka groups are small, only three to nine members, and females only breed every two years on average. If a group has only one breeding male and two breeding-age females—a common scenario—and an incoming male like Yellow Silver kills any unweaned infants, the next year the resident male may mate with one female and the immigrant male with the other. (Wright has genetic evidence that confirms this paternity outcome in two cases, she says.) Thus, both males sire offspring that next year, which is a year faster for both males than it would be if the unweaned babies had lived.In humans, child abuse is 10 times as likely in families where the biological parents are unmarried. This could indicate a link to behavior in other primates where one will protect one's own offspring at the expense of others'.
If you reasoned strictly from science, specifically biology, would you classify different kinds of human sexual relationships differently or would you lump them all together?