Haven't looked at the passage lately, but remember he's a sort of Hegelian, especially at this stage; everything has an objective standard that is also subjective. If I had to bet, without having studied it, I'd guess that he means that sex should, objectively, be a matter of mutual giving and taking and not imposition or domination -- actually the point you make in saying that sex with children is bad.
Marx himself was rather a sexual prude. He schtupped the maid and had a son by her, a son whom Engels generously raised as his own, but disapproved of Engels' Living In Sin with his girlfriend. The closest they ever came to a break was when Marx refused to comfort Engels in his mourning for her death, apaprently because he disapproved of their manner of living. Mill, btw, also Lived In Sin, and adulterously too, with his girlfriend Harriet Taylor, who practicallly coauthored some of hsi writings as wella s contributing her own. but that is another story. It would not surprise me if both men (Marx and Engels) had conventionally bigoted attitudes towards homosexuals. Sexual liberation was not so big for them as an issue -- unlike the liberation of women, which was important especially to Engels. But not so much the liberation of people to have all sorts of fun and love whom they like. That is a postmodern concept.
jks
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