If you want to get technical it just forced all nouns to be masculine, taking the masculine plural -es and genitive/possessive -'s. Even our definite article is the masculine form of what originally inflected for three genders, 'the' (m), 'thee' (f), and 'that' (n). It makes sense when you realize that just one inflectional class, originally the a-stem masculines which provided our plural and possessive, accounted for about 1/3 of all nouns just by itself in Old English. Once less marked classes started to merge with it, probably originally the neuter a-stems and other vocalic masculine stems, it just became the default.