Because Rockstar got enough lawsuits about people being able to have sex in GTA:SA with a mod. If they made it so that you could have sex without a mod in RDR, they would get fucked with a rake. A rake of lawsuits, that is.
EDIT: The thing is, this is a +18 game, so they should not fear to make able to have sex in the game. An adult content game with adult content.
Side note: If the did they would have to make it an AO game and 99% of stores would not carry the game.
Intelligent edit Edit
- Is there a POINT to having in game sex? Go watch a porno. I would rather see game space used for something interesting like mini games, challenges, etc, than letting someone have sex with hookers. If it is part of the storyline, fine, include sex in the game, but why make a big deal of not being able to buy hookers?