|It's true that the Hebrews, along with most ancient cultures, placed a high value on fertility and child-bearing, but I always understood same-sex marriage to be taboo, because it violates the sanctity of marriage (one man and one woman). If fertility was the driving force behind marriage laws, I don't think unfaithfulness or polygamy would have been frowned upon.
But why did the people who wrote the Bible put emphasis on 'marriage' (or some term for binding, long-term relationships) being between one man and one woman ?
The phrase 'sanctity of marriage' gets used a lot to defend a certain Judeo-Christian view, but the justification seems to be "it's in my Bible". And as we've seen, the Bible contradicts itself, is multiply translated through many languages (divine guidance through Chinese Whispers), and was written at a time when slavery was commonplace and women were the property of men.
It's important to ask 'why is this in the Bible ?' and 'Is this still relevent/appropriate in the 21st century ?'
We agree that production of children (and therefore believers) was important, therefore it makes sense to encourge heterosexual relationships.
(Of course, it was common in Classical Greece for men to marry women (to produce children and run the household) and to have male lovers (for the pleasure). Marriage with children, and homosexuality, are not necessarily exclusive.)
Unfaithfulness may lead to more children, but it is frowned up because the husband, who owns the money, cannot be sure that the children he's raising are all his, if his wife's been unfaithful. If he's been playing around, then another man could be raising his children and the husband won't get to pass on his property/name to them.
The men who historically owned the money/goods/land and women, liked to be in control of what they had. An unfaithful wife was not in their control. Husbands were encourged to be faithful themselves, and to treat wives well, so they would be less likely to stray.
Polygamy is not so much frowned upon; after all, it showed a man's wealth, and enabled him to have more children. However, multiple wives would have less of their single husband's attention, and might seek that attention elsewhere. If wives come from different families, then the husband ends up with duties to multiple sets of in-laws, which can be time-consuming and expensive. Keeping the numbers to one man/one woman makes things more managable.
So faithfulness is needed to control the bloodlines and inheritances of the children produced by heterosexual relationships. Marriage is a legally binding form of relationship so the husband (historically) acquired his wife from her family, making her children legally his.
This may seem cynical and not very 'holy', but they are good, practical reasons for why the men who wrote the Bible specifically defined marriage as 'one man, one woman'.
Are these still issues today ? To a certain degree, yes. Marriage gives relationships and children a certain legal status. Many couples manage just fine without, though. Unfaithfulness is still bad, for the reasons above, and also simply because it causes hurt.
Do we still need to encourage heterosexual relationships above homosexual ones in order to increase the birth rate ? Hell, no ! There's enough people on the planet already.
If the Bible was being written now, would it condone slavery ? No.
Would it need to abhor homosexuality, given the changes of the last few thousand years ?