Whether I would call it "sympathetically" or not, here is what Lewis wrote.

Here’s a fellow, you say, who used to come before us as a moral and religious writer, and now, if you please, he’s written a whole chapter describing his old school as a very furnace of impure loves without one word on the heinousness of the sin. But there are two reasons. One you shall hear before this chapter ends. The other is that, as I have said, the sin in question is one of the two (gambling is the other) which I have never been tempted to commit. I will not indulge in futile philippics against enemies I never met in battle.

(“This means, then, that all the other vices you have so largely written about…” Well, yes, it does, and more’s the pity; but it’s nothing to our purpose at the moment.)

This point is worth examining on its own. In the last few decades, Christian leaders have made a big issue of speaking out against gay rights and same-sex marriage. During the same period of time, they have taken a much softer line on no-fault divorce, fornication, and other offenses against the sanctity of marriage.

Lewis was an unusually persuasive apologist because he spoke humbly about struggles with sin in a way that others who struggled could relate to. This made him an attractive and trustworthy guide, and made the Gospel as he presented it believable and approachable.

Returning to Lewis’s words, toward the end of the chapter, he takes up the theme of homosexuality again (emphasis added):

The Wyvernians seem to me in retrospect to have been the least spontaneous, in that sense the least boyish, society I have ever known. It would perhaps not be too much to say that in some boys’ lives everything was calculated to the great end of advancement. For this games were played; for this clothes, friends, amusements, and vices were chosen.

And that is why I cannot give pederasty anything like a first place among the evils of the Coll. There is much hypocrisy on this theme. People commonly talk as if every other evil were more tolerable than this. But why? Because those of us who do not share the vice feel for it a certain nausea, as we do, say, for necrophily? I think that of very little relevance to moral judgment. Because it produces permanent perversion? But there is very little evidence that it does. The Bloods would have preferred girls to boys if they could have come by them; when, at a later age, girls were obtainable, they probably took them. Is it then on Christian grounds? But how many of those who fulminate on the matter are in fact Christians? And what Christian, in a society as worldly and cruel as that of Wyvern, would pick out the carnal sins for special reprobation? Cruelty is surely more evil than lust and the World at least as dangerous as the Flesh. The real reason for all the pother is, in my opinion, neither Christian nor ethical. We attack this vice not because it is the worst but because it is, by adult standards, the most disreputable and unmentionable, and happens also to be a crime in English law. The world may lead you only to Hell; but sodomy may lead you to jail and create a scandal, and lose you your job. The World, to do it justice, seldom does that.

If those of us who have known a school like Wyvern dared to speak the truth, we should have to say that pederasty, however great an evil in itself, was, in that time and place, the only foothold or cranny left for certain good things. It was the only counterpoise to the social struggle; the one oasis (though green only with weeds and moist only with fetid water) in the burning desert of competitive ambition. In his unnatural love affairs, and perhaps only there, the Blood went a little out of himself, forgot for a few hours that he was One of the Most Important People There Are. It softens the picture. A perversion was the only chink left through which something spontaneous and uncalculating could creep in. Plato was right after all. Eros, turned upside down, blackened, distorted, and filthy, still bore the traces of his divinity.