There are bits that are certainly true, bits that are probably true, and a lot of embroidering.
After nine years at Oxbridge (two as a staff member), I have never met anyone I knew to be titled/an aristocrat, although one student I matriculated with was rumored to be the son of a Middle Eastern oil baron.
There are a reasonable number of drinking societies (some of the have names like, er, the boat club, the darts team and the rugby club...), with initiations like drinking a pint of mud or swallowing a live goldfish. They tend to be full of students from normal backgrounds and are likely drinking societies or sports teams at any uni.
I know the sports teams I have been a member of have occasionally done obnoxious things (vomiting on the floor in restaurants, standing up on a chair to make speeches). That's not special to Oxbridge.
There are people with plummy accents who went to posh schools. Much like some of the students at the start of The Riot Club, they often try to hide it/play it down. I have occasionally been teased for having a Midlands accent - by state school friends from Bristol (because of course they have no accent) or my boyfriend (who is from the East End of London and is unintelligible to non-native speakers). Sometimes, 'posh' friends try to tell me they don't have an accent. I tell them they're wrong
There is money washing around, probably more than you'd see normally from a state school background - people in head to toe Jack Wills, people who think they're hard done by if the can't go skiing every year, an ex boyfriend who owned his own white tie; but it's not generally overt. The only time you might realise that your friend is Secret Posh (sounds and looks normal, actually rolling in it) is if you go to their 21st birthday party and Mummy and Daddy have pulled out all the stops - balcony of the Shard hired for the night, marquee their 10 acres of Surrey grounds, Tudor mansion booked out for the weekend...
Sexism is there are all universities, men with a sense of entitlement simply because they're men. Oxbridge isn't special in that regard.
As for coke and prostitutes - I've been told that goes on, but I haven't seen any evidence of it personally.
But the Bullingdon Club and dining societies like it ARE real. When I started at Oxford in 2006, the student papers reported that an un-named dining society had smashed up a pub near Oxford and their parents had paid for all the damage (£££££) to be repaired by cheque. Pretty disgusting really. But how often do young men and women who aren't rich commit criminal damage for the sake of it?