You were not misinformed. Indeed, these very passages actually demonstrate that pederasty was widely practiced in Sparta. Elsewhere, Plutarch openly states that when Spartan boys reached puberty, they became available to older male lovers (Life of Lykourgos 17.1). There are several examples of famous historical characters who were once either lover or beloved in pederastic relationships (such as the fourth century BC king Agesilaos). Modern scholars do not doubt that man-boy love was very common in Classical Sparta. It has even been suggested that the only way to become a member of one of Sparta's tent groups upon reaching adulthood was to be the beloved of one of that tent group's existing members, making pederasty a fundamental element of Spartan social organisation.

However, the two authors you've cited (and it appears Plutarch is building on Xenophon, so that there is really only one source) are trying to paint an idealised picture of Sparta, in which every element of their way of life serves to create better citizens. They therefore chose to portray pederasty as a wholesome element in a boy's education, while of course denying that any part of it is governed by the rampant homosexual desires of adults.

It is worth noting that pederasty was primarily practiced among the leisured elite (which in Sparta meant all full citizens) and that there was a definite political element involved: being desired by a powerful lover helped a beloved's family along, while acquiring a beloved from an influential family could improve the status of the lover. This political dimension likely resulted in a choice of boy lovers that might not be easily explained through physical attraction alone. Yet it would of course not be good form to claim openly that the whole thing was just a power play. Much better to pretend that the boy's 'soul', his good character and impressive qualities, were what drove the older lover to take the beloved under his wing. This complimentous approach was a socially acceptable way to mask both homosexual desire and political machinations.

In addition, Xenophon seems to have been generally reluctant to accept the physical dimensions of pederastic relationships, which fits into a discomfort with raw instincts like lust among wealthy and educated men throughout Classical Greece. They liked to portray themselves as sensible and moderate; falling madly in love with boys was not easily justified, even though it was clearly common enough, and entirely socially acceptable. Whatever they may have said, though, we would be very naive to think that all these pederastic relationships were indeed entirely pure and platonic.

As for sexual relationships among "soldiers", there were no soldiers at Sparta, which had no professional army. Adult males were not supposed to engage in homosexual relationships - the institution of pederasty only sanctioned unequal man-boy ties - but it seems, again, naive to assume it didn't happen. If nothing else, it would have been common for man-boy relationships to continue once the boy had reached adulthood, since the lover was usually only a decade or so older than his beloved, and they would often have ended up being part of the same tent group, eating and sleeping together every day for the majority of their adult lives.

Much of this post is based on K. Dover's 'Greek Homosexuality and Initiation', in his The Greeks and their Legacy (1988), and its discussion in S. Hodkinson's 'The Development of Spartan Society and Institutions in the Archaic Period', in Mitchell and Rhodes' The Development of the Polis in Archaic Greece (1997). The main handbook on this subject is still K. Dover's Greek Homosexuality, newly reprinted with discussion of more recent scholarship by Bloomsbury (2016).

I've seen very few things that suggests that Sparta embraced homosexuality as a norm and much that suggests that homosexuality actually was frowned upon.

The Spartan's, as was common in Ancient Greece practiced the erastes system, a adult man would serve as a guide and help a boy into adulthood. the purpose of this bound was pedagogical, but it seems it wasn't that uncommon for this relationship to also have a erotic or sexual dimension. So lover and beloved doesn't mean what it sounds like. And the nature of the relationship varied a lot as Xenophon notes. To compare, Sparta used the terms (eispnelas) inspirer (aites), listener for this relationship, Which isn't quite so suggestive. As you have noted, the ancient sources claims that in Sparta this bound were intended and required to be pedagogical.

So were that the case? A lot of things suggests it was. The famous pederasty in Greece weren't due to a greater numbers of homosexuals. but rather as in works in parts of the Middle East and Afghanistan today. Females were from cradle to grave kept out of sight of men, in special quarter of the house, Greek men married late when they accumulated enough wealth to support a family. Meanwhile boys tended to serve as a sexual substitute.

Sparta on the other hand was infamous for their indecently dressed women that roamed freely among their men from childhood. Plutarch said they swam together and appeared naked in public. Spartan men also tended to marry younger since the land granted to them by the state provided for their family. As Euripides have Peleus say in Andromache : Not even if she wanted to could a Spartan woman be chaste. They leave their houses in the company of young men, thighs showing bare through their revealing garments, and in a manner I cannot endure they share the same running-tracks and wrestling-places.
And marriage and babymaking were serious business. Xenophon tells us that unmarried men had to run around naked in winter mocked by the women. Younger men with a heir didn't rise for older men without. Aristotle says Spartan had laws that excused them of taxes if they had 3 sons and all state obligations if they had 4. Despite this, the number of Spartiates continued to decline.

Aristole also have a rather interesting explanation to why Spartan women have such freedom, lack of homosexuality:

And this is what has actually happened at Sparta; the legislator wanted to make the whole state hardy and temperate, and he has carried out his intention in the case of the men, but he has neglected the women, who live in every sort of intemperance and luxury. The consequence is that in such a state wealth is too highly valued, especially if the citizen fall under the dominion of their wives, after the manner of most warlike races, except the Celts and a few others who openly approve of male loves. The old mythologer would seem to have been right in uniting Ares and Aphrodite, for all warlike races are prone to the love either of men or of women. This was exemplified among the Spartans in the days of their greatness; many things were managed by their women.

Aristotle, Politics, book 2 chapter 9

So there really doesn't seem to be a reason for homoeroticism, rather the opposite in fact.

However, the two authors you've cited (and it appears Plutarch is building on Xenophon, so that there is really only one source) are trying to paint an idealised picture of Sparta, in which every element of their way of life serves to create better citizens. They therefore chose to portray pederasty as a wholesome element in a boy's education, while of course denying that any part of it is governed by the rampant homosexual desires of adults.

So why would Xenophon lie? He himself says that this is acceptable behaviour in many Greek poelis so much so that many won't believe him when he says it's not the case in Sparta. Since he consistently praise Spartan society it's rather inconsistent of him to lie about their acceptance of homosexual childsex, He clearly states that Lycurgus viewed it as abomination comparable to incest, shouldn't the Spartan court that protected him be rather upset about putting lies in Lycurgus mouth castigating all their perfectly acceptable homolove? Or do you mean that the Spartans, famous for their obedience to the laws, systematically practiced homosexuality in defiance of Lycurgus edicts? I've yet to see a compelling reason for why Xenophon would lie about this, he tended to leave out less flattering facts about Sparta in his writing, he could easily have done so here as well.

In addition, Xenophon seems to have been generally reluctant to accept the physical dimensions of pederastic relationships, which fits into a discomfort with raw instincts like lust among wealthy and educated men throughout Classical Greece.

Which would make sense if the Spartan culture he admired viewed this as unacceptable. Also Xenophon had both his sons sent through the agoge, so if it was a hotbed for homosexuality he can't have been that bothered.