I could see Voldemort being a rapist, taking what he wanted by force. I don't think he would want to have sex with someone who was willing, think that would have turned him off. Could also see him killing the women afterwards once he left school. I think snape could have died a virgin, because he never got over Lilly, although it's possible he had the odd fling to try to get over her.
I think each year has about 140 pupils, but not x amount from each house. Would Hufflepuff have a lot more pupils than the rest? And Slytherin much less?
There are certainly other ways than the killing curse to magically kill people. Molly Weasley killed Bellatrix with an unidentified curse. I think she knocked the wind out of her with one spell then smashed her to pieces with something like reducto.
The gold plates are probably Goblin crafted, or magically altered, so don't go by the same rules as muggle golden plates 
As for contraception, I imagine they have some sort of potion that can be used, (and obtained from Madam Pomfrey) and potion shops/ St. Mungo's etc. And that Lilly forgot to take it for whatever reason. Molly Weasley must have just loved babies/kids and wanted a big family. Or just kept trying until she had a girl (remember when Ron "kills" the horcrux, it plays on his fear his mother would have loved him more if he'd been a girl).
I think the snake on taps in chamber of secrets would have been noticed, but as no one could speak parseltongue they would have had no idea that it opened anything. Probably just put it down to normal graffiti.
I was thinking the other day about how ironic and brilliant it was to have Narcissa Malfoy (because of her love of Draco) save the day. And how people always seem to overlook the fact she did save Harry who then saved everyone else.