It is my understanding that if the court had not made this determination, it might have had to uphold the defense's claim to right of privacy, and the defendants would have have been acquitted on appeal.

It is worthwhile quoting the entire relevant part of the judge's opinion, for those who don't want to spend time reading the source:

First, the decision to take photographs and to keep a record that may be shown to people in the future weighs against a reasonable expectation of privacy. See [....]

So, he equates privacy with deliberate secrecy. That's not a presumption of privacy. Note that I'm commenting on common reason, not on whatever the relevant case law may be.

Second, the photographs which were taken were shared by the two minors who were involved in the sexual activities. Neither had a reasonable expectation that the other would not show the photos to a third party. Minors who are involved in a sexual relationship, unlike adults who may be involved in a mature committed relationship, have no reasonable expectation that their relationship will continue and that the photographs will not be shared with others intentionally or unintentionally. One motive for revealing the photos is profit. Unfortunately, the market for child pornography in this country, according to news reports, appears to be flourishing. See, e.g., [....] These 117 sexually explicit photographs would undoubtedly have market value.

(My emphasis.) So, he says that because they are teenagers, their relationship cannot be "mature and committed", and so we must assume that one of them will in the future hurt the other for money (not even in revenge or somesuch).

In addition, a number of teenagers want to let their friends know of their sexual prowess. Pictures are excellent evidence of an individual’s exploits. A reasonably prudent person would believe that if you put this type of material in a teenager’s hands that, at some point either for profit or bragging rights, the material will be disseminated to other members of the public.

Distribution of these types of photos is likely, especially after the relationship has ended. It is not unreasonable to assume that the immature relationship between the co-defendants would eventually end. The relationship has neither the sanctity of law nor the stability of maturity or length. The subjective belief of these co-defendants that the photos might not be shared is not dispositive. In fact, the defendant in this case expressed her concern to law enforcement that her co-defendant might do something disagreeable with the photographs.

The mere fact that the defendant may have subjectively believed that the pictures would remain private does not control; it is whether society is willing to recognize an objective expectation.

(My emphasis.) IOW: each of them is capable of harming the other in the future. Because they are teenagers, we assume they will do so. We explicitly disregard their trust in one another; the relevant point is that we, the society, do not trust them. Therefore we will punish both of them proactively by withdrawing their right to privacy.