Marriage is simply formation of bonds between opposite sexed people, in order to attain stability. It is no doubt that we are born with deficiency, hence why male sex cells carry half instructions for the formation of a young one, while the other half is carried in the female ova.
This is the reason why we are incomplete by ourselves, we are completed by our spouses. This is the same as in the chemistry of atoms. They need to form bonds in order for them to become stable.
The strongest bond is normally the ionic bond. This forms the basis of heterosexuality. A metal forms a bond with a non- metal. The bond is stronger to break and the compound formed needs to be subjected to extreme conditions for it to decompose.
In biology we have X and Y chromosomes, whereby XX would mean female and XY be male. This by lame explanation would mean that the initial Xs in both is just for our being and the latter X and Y would be the determination of our gender.
If by any change two homosexual were to be fused, there would be high chances of extinction to the other gender or even the parent generation as the would be some offspring of unknown being.
In the case of formation of offspring, we need both boys and girls/males and females produced.
Giving birth is the continuation of human chemistry, which completes the matrimony. A gift of nature from creation.