When you researched if they existed back then, did you also look at their origins? Bikinis were invented in the Cold War when everyone thought the world was about to end. They were having a fashion show and someone came up with the idea. The people in charge said it was too scandalous, but the designer said, "hey, the world might end. If it does, no one cares if we put out a scandalous swim suit. If it doesn't, it might catch on!"
Lo and behold, the world didn't end, and the bikini caught on!
*Is a total history buff*
When did Lily and Snape ever go to the beach together?? I don't remember this at all!
I can just read his mind, "I hate you, Lily. You've got me out here all naked and cold, not to mention the gray skies." Then he looks at her... "But I still love you, because you've got YOU out here all naked. So that just made my day a whole lot better!"
Of course he would never say that out loud. But even though he's all shy and whatnot he's still thinking about Lily. Cause he's in love... <3 Aww...