During United States Supreme Court oral arguments today (April 28, 2015) in the case of Obergefell v. Hodges, "biology" and its cognates were uttered 27 times in relation to whether or not there is a federal constitutional right to same-sex marriage. John J. Bursch, Special Assistant Attorney General for the State of Michigan, opposing same-sex marriage, repeatedly argued variations of the following mantras: "[W]e want to encourage children to be bonded to their biological mother and father" and "[W]e want to forever link children with their biological mom and dad when that's possible." Justice Elena Kagan, echoing several other justices, offered this challenging retort: "[I]t's hard to see how permitting same-sex marriage discourages people from being bonded with their biological children." In addition, it was noted repeatedly that many married couples either adopt children born to other parents - as Chief Justice Roberts and his wife have done - or have no children at all.
Bursch leaned heavily on "biology" to support his moral arguments against same-sex marriage. Yet, biology and morality make strange and unmarriageable bedfellows. Biology can, indeed, shine considerable light on a number of social issues discussed in the oral arguments, including bases for sexual preferences, means and patterns of procreation, and even why "fathers with the benefits or the requirements of marriage walk away from their children". However, pair-bonded humans reproduced successfully and prodigiously for aeons before the invention of the institution of marriage. So, any necessary connection between marriage, bonding, and "biology" remains unsubstantiated at best.
Biological evidence can be useful to support legal arguments. The use of DNA evidence to convict and exonerate criminal suspects may be a success, but the attempt to use "biology" to deny marriage rights to same-sex couples is a failure.