That is the point of marriage, and any thinking about protecting, reforming, or extending the institution must bear it in mind. I am guessing that same sex marriages generate some of the same dramas as traditional marriage. While children are not usually the issue (forgive pun), jealousy and infidelity probably feel much the same. If marriage is to work for anyone, it needs a social sanction. We ought to demonstrate the same indignation toward adultery as we due towards racism and other forms of intolerance. If you really believe in gay marriage or any other kind of marriage, that indignation is what it means [Ken Blanchard, "On the Meaning of Marriage," South Dakota Politics, 2009.05.16].
Indeed, our state which has outlawed gay marriage by constitutional amendment imposes little if any penalty for adultery. And a spouse who has an affair commits a much more direct assault on the sanctity of those forgotten wedding vows than any two men or two women who choose to say those vows to each other.
Perhaps Senator Thune will as vocally reject the leadership of adulterers like Newt Gingrich (not to mention serial divorcés like Rush Limbaugh) as he vocally rejects the elevation of homosexuals, faithful or not, to the Supreme Court.