If you've read the books, Harry bathes or washes two times in the entire saga, nobody pees, poos or farts, Harry is told to wash his hair the night before going to the ministry in The Order of the Phoenix. Maybe there's a spell or enchanted object that cleans you without bathing?
The sex part is a bit weird but you must not forget the Weasleys. It's just not what you want to graphicllygraphically imagine Molly and Arthur Weasley having sex. It's not very sexy is it? But they do have fiveseven children...
The rest of the wizarding world seems to be quite LOW on libido, for even after Hogwarts they only have one or two kids, even when they have the money...
I think if you're very busy and generally having fun, sex is the last thing on your mind, magic is just cooler than what we do at school, specially when we can't choose what we like and we have to deal with 10 if not more of school years compared to Hogwarts' 7. Not that it doesn't happen, I'm sure it does... but then what if they got pregnant? Where would they get condoms? If they are pure bloods where would they get muggle money to buy said condoms, is there a spell that makes girls not ovulate or boys sperm go puff?
And there is the fact that ALL rooms are shared, that puts a damper on masturbating right?
My question is how do witches deal with periods? Can they do "Evanesco" at home if they're muggle born ?