I imagine that Sherlock would think being in a relationship is trivial and boring. Personally, I believe that he is asexual. I don't think he veiws people sexually, but rather he sees them as puzzles to be solved and taken apart again. He doesn't need physical intamacy because he is satisfied by his own mental pleasures. If nothing else, Sherlock views sex as an unecessary distraction and an emotional vulnerability. As we all know, caring is not an advantage.

As far as Irene Adler goes, I don't feel like Sherlock is physically interested in her. Everyone else is so easy to read, but she's a challenge. He almost sees her as an equal, which is why he goes to great lengths to impress and protect her. He cares about what she thinks of him and what happens to her because she intrigues him.