"At the heart of liberty is the right to define one’s own concept of existence, of meaning, of the universe, and of the mystery of human life." [Planned Parenthood v. Casey, 505 U.S. at __; quoted in Lawrence, 539 U.S. at 574]
I in turn noted that this passage wasn't all too removed from Locke; Van Dyke disagreed. I've never thought much about the "mystery of life" passage and admit that it's a bit mushy. But if I were to argue how Locke's ideals might support such a worldview, without the mushiness, I'd turn to Locke's self ownership principle.
Harvey Mansfield discusses this dynamic in this article, where he writes:
For Locke, then, the harmonizing of liberty and virtue begins from the harmonizing of liberty and religion. In the face of the apparent fact that the Christian religion tells men how to live, he must show, if he can, that it actually permits them to live in freedom. How does he proceed?
Locke gives two descriptions of the character of men in their fundamental relation to liberty. He says that they are the “workmanship” of God, that men are “his [God’s] property” and so belong to God; but he also says that “every man has a property in his own person.” These appear to be directly contrary because the “workmanship argument” (as it is called by Locke’s interpreters) would make man a slave of God whereas the idea of property in one’s own person sets him free to do with himself what he wishes. Thus Locke says, in accordance with the former, that men have no right to commit suicide (“everyone is bound . . . not to quit his Station wilfully”). But in accordance with the latter, though saying nothing directly about a right of suicide, he pronounces that in the state of nature, man is “absolute lord of his own person and possessions.” Yet Locke does not make a point of the contradiction between these two descriptions. It is rather as if he had forgotten what he said earlier or perhaps lost his train of thought. Yet Locke does not seem to be a woolly-minded fellow, and his reputation shows that both his friends and his enemies take him seriously. His political thought typically contains contradictions, of which this one is perhaps the most important, but he leaves the reader to do the work of establishing the contradictions and working out their implications. In this case and in other cases, Locke does not leave the contradiction as flat as I have reported it; he teases readers with possible routes by which it might be harmonized. But most of all, Locke lets readers do their own harmonizing by allowing them to combine two things they want to believe. Almost all of Locke’s readers would want to believe in the truth of Scripture, and many of them would like to think, or might be persuaded to think, that their belief is compatible with, or even entails, the notion of liberty that Locke sets forth.
The difference between belonging to God and belonging to yourself is not a small one....
Some folks have observed that Mansfield makes too much of the difference, that it is easily reconciled by noting God gives men a "leasehold" or "life-estate" over that which ultimately belongs to Him. Fine. However, even so Locke's ideal still demands that the individual, not the government purporting to "impose morality" have the ultimate Earthly say on how to live his life, in all but a few cases.
By way of analogy, the debate over Romans 13. That passage says:
Let every soul be subject unto the higher powers. For there is no power but of God: the powers that be are ordained  of God.
For rulers are not a terror to good works, but to the evil. Wilt thou then not be afraid of the power? do that which is good, and thou shalt have praise of the same: 4 For he is the minister of God to thee for good. But if thou do that which is evil, be afraid; for he beareth not the sword in vain: for he is the minister of God, a revenger to execute wrath upon him that doeth evil.
Okay, so let's say you have a tyrannical ruler, who is un-godly and doesn't quite seem "not a terror to good works, but to the evil." The context of the passage suggests that Paul was telling believers to submit to Nero. Indeed the dominant interpretation in the history of biblical orthodoxy teaches this. And Nero was indeed a pagan tyrant who ruthlessly persecuted Christians.
This dominant biblical interpretation teaches Nero was indeed a ruler ordained with God given authority. Therefore, the Earthly buck stops with him. Obviously his actions displeased the biblical God. And ultimately it's God's authority alone (not "the people's") to punish Nero for his evil behavior [and indeed Nero met an ultimately demise].
Another interpretation might suggest that since Nero didn't seem to be "not a terror to good works, but to the evil," he wasn't a "ruler." This interpretation has been a minority throughout Christendom, but has become more popular in the age of revolution where Christians attempt to biblically justify revolt against tyrannical leaders. But ultimately the early church fathers, medieval Roman Catholic Church, and first Protestant reformers disagreed with it. Indeed Calvin argued that "we must honour [even] the worst tyrant in the office in which the Lord has seen fit to set him" and "if you go on to infer that only just governments are to be repaid by obedience, your reasoning is stupid."
Back to Locke. According to him, an individual owns himself, but ultimately belongs to God. So where does the Earthly buck stop? Why, with the individual of course. So if I own myself, I can do whatever I want with my body, as long as it doesn't interfere with your right to do whatever you want with your body. And if I violate the natural law or God's will, it's up to God alone, not you or the collective to punish me for it. And indeed, individuals suffer consequences from drug and alcohol abuse, irresponsible promiscuous sex and the like. Someone who transgresses God's law by drinking too much and dying prematurely of cirrhosis of the liver suffers from violating God's law without the need for human intermediaries to step in on God's behalf to prohibit the behavior because it violates God's law. The individual owns himself, the earthly buck stops with him and, similar to Romans 13 and the tyrannical leader, it's up to God alone to punish him for using his body in a way that might offend Him.
[Note: Yes, of course there is HUGE tension between the traditional understanding of Romans 13 and Locke's self ownership principle. America's Founders, it should be noted, chose to follow Locke.]
So, how does this play out in 20th Century American constitutional jurisprudence [and remember, Locke, for good reason has been termed "America's philosopher"]? Justice Blackmun's dissent in Bowers v. Hardwick (the case holding states could criminalize sodomy, overturned in Lawrence v. Texas) held “the concept of privacy embodies the `moral fact that a person belongs to himself and not others nor to society as a whole.’” In Slouching Towards Gomorrah, Robert Bork responded: "There are 'moral facts,' but that is certainly not one of them." p. 104.
Well, John Locke is the author of the "moral fact" that an individual belongs to himself.