These are 2 entirely different things let me put it in comparison for the both of you :
Fucking a prostitute in game =/= fucking a prostitute in real life. You cannot simulate the "pleasure" you receive irl through a game, therefore any child coming across this kind of stuff wont even know what it really is.
Smoking weed in a game =/= smoking weed in real life. You need LUNGS, a MOUTH and a JOINT to actually know what weed is like, you have none of these ingame because you are in a virtual world with virtual people and therefore you dont get any taste of what it's really like. Again, children that come across their character using weed won't even understand what it is.
GAMBLING IN A GAME = GAMBLING IN REAL LIFE. In real life casinos you have video poker, blackjack, poker so on and so forth. Only real thing you are missing are the cards which you cannot touch physically but that's not even relevant seeing as their only purpose is to show you "your hand" which is VISUAL. Anything that's strictly visual can easily be replicated electronically which is why the concept of online gambling/betting exists and is very functional. Blackjack/poker in a game = Blackjack in real life because the only thing you are missing is a real dealer which isn't even a problem because the dealer can easily be replaced with AI for the exact same effects. Don't get me started on video poker or roulettes as those can be very EASILY made electronic.
My point is casinos in real life can easily be made online as it requires no interaction from you, at least not physically. You don't consume anything, you don't have any sex with anything - its all simply visual. The games you see in real life can also be made virtual. A simple google search of "online casinos" gets you back with A MILLION RESULTS. Would you have the same luck by googling "online prostitute" or "online weed"? I don't think so. I rest my case.