Sorry, but there is no biblical prohibition on pedophilia, and the bible approves of relationships that we would define today as child sexual abuse. It also is silent on polygamy, which involves multiple partners. There are very strong commands against adultery, but given the property view of women one finds in the bible, that is neither surprising nor enlightened.
Regarding pedophilia, child abuse and the like, Jesus brought a child before the disciples and said this:
Matthew 18:6 But whoso shall offend one of these little ones which believe in me, it were better for him that a millstone were hanged about his neck, and
that he were drowned in the depth of the sea.
And regarding polygamy, the intent of God for one man and one woman to become one flesh is clear from Jesus' words:
Matthew 19:4-6 4 And he answered and said unto them, Have ye not read, that he which made them at the beginning made them male and female,5 And said, For this cause shall a man leave father and mother, and shall cleave to his wife: and they twain shall be one flesh?6 Wherefore they are no more twain, but one flesh. What therefore God hath joined together, let not man put asunder.
Regarding women being "property", Paul says:
Galatians 3:28 - There is neither Jew nor Greek, there is neither bond nor free, there is neither male nor female: for ye are all one in Christ Jesus.
I do not deny the headship of a man in a family, nor certain roles that the Bible gives to men and women, but the Bible also stresses the intrinsic equality of man and woman in Christ, which was pretty revolutionary for its time and today is overlooked as a no-brainer.
The problem is that your "design" theory ignores the realities of evolution by natural selection; there is no ascertainable intelligent design behind human biology, including our reproductive systems. And reproductive fitness is quite complex; it is possible that homosexuality can be explained in evolutionary terms by way of kin selection, which would provide ample "justification" for homosexuality.
The OP asked a question that rested on theistic premises, I am explaining from the Christian worldview why God can disapprove of homosexual relationships and approve of heterosexual ones. I am not trying to argue for design, these things are assumed within the Christian worldview.
And yes, "it is possible" to explain anything by any means, really, I am yet to see evidence for said claim.
The other problem with the design argument you are making should be obvious: Oral sex is not condemned, nor is anal heterosexual sex. Yet both actions involve non-reproductive sexuality; oral sex is in many if not most ways less risky than vaginal intercourse (with respect to HIV transmission, certainly). And yet another problem is easy enough to ponder: Anal sex can clearly be pleasurable, which suggests that there may be more to the story here, even if you accept intelligent design and ignore the clear evidence of evolution by natural selection.
Anal sex is dangerous considering the potential bacterial infections, for heterosexuals and homosexuals alike, I don't see the appeal of the poop-tube, I don't think there is "more to the story". Heterosexual relationships have the possibility for reproduction, I said, not that everything will cause it. We have many pleasures that we seek to gratify in different ways, not all are correct, we and Creation are fallen.
If the Christian god condemns homosexuality while claiming that all that is created is good, and given that we know that homosexuality exists in nature quite apart from humans, the very existence of homosexuality would seem to call the existence of the Christian god into question.
If the Christian god condemnns cannibalism while claiming that all that is created is good, and given that we know that cannibalism exists in nature quite apart from humans, the very existence of cannibalism would seem to call the existence of the Christian god into question.
I can substitute many terms in here which appear in nature, seeking to answer moral questions with nature is shaky ground. Creation is fallen, it was created good but mankind rebelled and was corrupted.