I have been rather astounded to find that other more formal etymological resources agree. The possibility had never even occurred to me that it might come from something other than mas "male" + turba- "provoke, agitate, disturb, excite". When I read your question, and found that a standard 19th-century dictionary agreed with Etymonline, I presumed it was merely old-fashioned prudery that would require it to come from man(u)- "hand" + stupra- "defile".

But it seems that most of the linguists agree that it does. (There are a couple of other competing derivations, but they're pretty far-fetched.) The OED, the Oxford Latin Dictionary, and Henriksen's commentary on Martial book 9, all agree on this. Michiel de Vaan's etymological dictionary doesn't include the word, which is a pity.

But then, it is an extremely rare word. It appears only in one ancient author, the epigrammatist Martial (Epigrams 9.41.7, 11.104.13, and an agentive form at 14.203.2). Henriksen takes it that, if the word does in fact come from man- + stupra, the sound shifts (loss of n and lengthening of a, metathesis of pr) mean that it must be an obscure archaic word that Martial is reviving, and not a neologism.

So whatever else, the thing about "defiling" is not a reflection on Roman sexual customs. If the word comes from stupra-, the only surviving author to have used the word may not even have been aware of that fact. But Etymonline does reflect the linguists accurately.