The three authors that you mention are actually terrible examples of this sort of thing, because their manuscript traditions were largely dead in the Middle Ages. Sappho's manuscript tradition was already dying in late antiquity as fewer and fewer people bothered learning Aeolic--by like 1200 or so her manuscripts were lost even among Byzantine scholars. What we know of Sappho consists of papyrus fragments and quotations of her work, and we've only got a couple of complete poems. Rutilius only exists in a fragmentary state, descending from a single (now lost) copy in a monastery at Bobbio in the late Middle Ages, and the assumption has long been that Rutilius just wasn't read very often (he's really not very good, after all), although in the 70s there was the discovery of about 40 more lines in a fragment. Catullus was almost entirely lost by the time of the Antonines, having fallen starkly out of fashion in the 2nd Century, and he survived only in a single manuscript that was discovered in the 14th Century.

Medieval scribes were, more often than not, quite literally fanatical about preserving the entirety of the body of work available to them. Important manuscripts and teaching texts were naturally copied in abundance, but with very few exceptions scribes did not intentionally neglect manuscripts that they had. Lots of texts for which we have loads of copies aren't exactly in keeping with medieval church doctrine, yet they were copied extensively anyway. In point of fact the vast majority of texts that have been lost were either lost or on their way out in late antiquity. And medieval scribes were hardly the fuddy-duddies that people seem to think they were. The omission or revision of...morally questionable passages in classical literature is generally not a feature of medieval copyists, it's something that really pops up in the 18th and 19th Centuries, which is also when editions of Sappho's and Catullus' poems started to often get screwed with in an effort to either eliminate vulgarity or explain away Sappho's obvious homosexual tendencies. In fact, medieval scribes often loved this sort of thing. For example, there's actually a passage in a single manuscript of Apuleius that is generally regarded as being a medieval interpolation (though a handful of editors consider it genuine), known as the spurcum additamentum. The spurcum additamentum interpolates a lengthy exposition on donkey genitals that can only be called pornographic (it's about jerking a donkey off and is one of the filthiest passages of Latin in existence) into the text as a sort of expansion of what's actually going on and doesn't seem to have been added for any other purpose than the amusement of the copyist. So much for morally-straitjacketed copyists!