You may think you're protecting your child, which I completely understand because parents only want the best for their children but just think.
By the time kids reach 13 they are already familiar with all the swearing, violence, and sexual themes contained in this game. As long as your kid isn't living under a rock, nothing will be new to him. As parents, you may not want to think like that but it is true and you know it. In all there are actually very few sexual scenes contained in this game. Your character can not actually physically engage in any sexual activity, but some can be witnessed in the cut scenes.
Most people skip the cut scenes anyway though cause after a while they just become boring. There is a decent amount of swearing too, but nothing too profane and again, its all in the cut scenes. Use of alcholol and drugs should not even be considered a problem in this game, unless of course your kid had never seen anyone pick up a beer and take a sip.
In singleplayer you can go to the bar and waste your money on whiskey that makes you pass out after a few drinks, but most people aren't stupid enough to waste their well earned money on booze, even in a video game.
Even after all of this if you still don't want your kid playing single player, you should still let them play multiplayer. Multiplayer has no cutscenes, so the swearing, drinking, and sexual scenes are all removed.
The violence is still there, but the violence in the game ia no worse then that of Call of Duty or Battlefield. While playing online, you can also form a posse with your friends and execute fun missions without any of the crape people keep mentioning. It is a fun experience! Overall, I give this game 5 stars because it allowed me to be the cowboy I always wanted to be back when I was filled with endless imagination. The depiction of the old west is beautiful, and actually quite breath taking at times.
As for the appropriateness of the game, once your child turns 13 be should be able to handle it. Unless your kid is a sociopath who is going to copy everything he see's in the game, you should be good: If it wasn't for a few unnecessary cut scenes this game would easily be rated T.
I believe those scenes were intentional though just so the game could be rated M, because how many extremely popular T games can you name? Still trying to think of some? That is why in my opinion the rating system in the US is crap because companies intentionally rate their games M to atrract a wider audience. It is better to read these things because the ratings cant be trusted.
Well if anything I hope I managed to convince a few people to purchase this game and a few a parents to allow their kids to purchase this game. Do not deprive them of the fun!