Some things just need to be grounded in biological fact and consequences.
Friendships do not produce children, and we want to have one model of sexuality for both straight and gay relationships.
Ambiguous parentage leads to strife over which potential parent (or parental clan) gets legal control of the child's living conditions, which can easily scar the child, especially if it happens before the 'age of reason' where the child can logically absorb the two parent's agendas and play along.
In previous social arrangements tied even more directly to biology, for instance, Attic Greece, that has meant that one could have it both ways, but only if one split the sexes the right way. Since gay relationships don't produce children to fight over, only property, one could have (as did Sappho) a spouse, and a number of lovers of your own sex.
But our modern taste for consistency and equality means that this double standard offends us, and we want everyone to be monogamous for extended periods.
In a wealthy modern technological society, this may no longer be so relevant, as we have genetic means to ascertain fatherhood, and we have strengthened our social conventions for both separated parents and effective adoption when the parents choose to be separated from their responsibility. We are almost to the point where the resources consistently exist to make those inefficient systems work to an acceptable degree. So, given a tiny bit more tolerance for complexity, and a little more money, we might slowly reduce this bias.
(Of course, as a gay man, I see this deduction as a bias because it is a consequence of the presumption that the straight model is the right one, and that sex is primarily about children.)
But our moral code is fixed much earlier in time.