According to Alex Abramavich: 'One day, midway through a course on The Waste Land, someone (possibly me) asked about the "silk handkerchiefs" – "testimony of summer nights" – floating in Eliot’s Thames. Those handkerchiefs had probably been used as prophylactics, my old thesis adviser Stanley Sultan said.'
I can't believe so. Which is to say, I can imagine somebody using a silk handkerchief as a prophylactic (though it's not going to be a very reliable one); but I can't imagine anyone being so profligate as to chuck one in the river after using it. Silk is very expensive. Silk can easily be washed. This interpretation is, I'd say, bogus.