It just is.  Martin Davie writes:

‘This is a point that is well made by the American writer Michael Brown in his book Can you be Gay and Christian?  He asks the question why there are only a tiny number of biblical verses that directly address the issue of same-sex sexual relationships.  His answer to this question is to draw an analogy with a book of recipes for sugar free puddings that has an introduction that explains why sugar should be avoided. The book would not need to constantly say ‘no sugar’ because this would be the point of the book. In a similar way, he says:

The Bible is a heterosexual book, and that is why it does not need to constantly speak against homosexual practice. It is heterosexual from beginning to end, and my heart truly goes out to ‘gay Christians’ trying to read the Bible as ‘their book.’  For them it cannot be read as it is; it must be adjusted, adapted, and changed to fit homosexual couples and their families. In short ‘gay Christians’ must read God-approved homosexuality into the biblical text since it simply isn’t there.

And this is the pattern throughout the entire Bible in book after book.

Every single reference to marriage in the entire Bible speaks of heterosexual unions without exception, to the point that a Hebrew idiom for marriage is for a man ‘to take a wife.’

Every warning to men about sexual purity presupposes heterosexuality, with the married man often warned not to lust after another woman.

Every discussion about family order and structure speaks explicitly in heterosexual terms, referring to husbands and wives, fathers and mothers.

Every law or instruction given to children presupposes heterosexuality, as children are urged to heed or obey or follow the counsel or example of their father and mother.

Every parable, illustration or metaphor having to do with marriage is presented in exclusively heterosexual terms.

In the Old Testament God depicts His relationship with Israel as that of a groom and a bride; in the New Testament the image shifts to the marital union of husband and wife as a picture of Christ and the Church.

Since there was no such thing as in vitro fertilization and the like in biblical times, the only parents were heterosexual (it still takes a man and a woman to produce a child) and there is no hint of homosexual couples adopting children.  

The Bible is a heterosexual book, and that is a simple, pervasive, undeniable fact that cannot be avoided, and, to repeat, this observation has nothing to do with a disputed passage, verse or word, it is a universal, all pervasive, completely transparent fact. (pp.88-89)