What I think, or for that matter what anyone thinks, about casual sex versus sex between two people who are in a "serious" romantic relationship, isn't really as important as what YOU think. From the sound of things, it seems like you've thought about the physical side of things, which is a great start.

Sex can mean a variety of different things to different people, and many people find that they don't always have to be "dating" or "in love" with someone in order to share the fun of a sexual experience with them.

However, it's hard for sex between people to be absolutely "casual." It can also be difficult to predict how you may feel about yourself or your friendship after the sexual experience. Sex can be a very intimate act and may complicate even the strongest friendships; some people find it difficult to be so close to somebody on one level and maintain their distance on another.

Sometimes, though, people also find a sexual relationship with a friend gives them a safe place to learn and experiment with possibly less pressure, awkwardness or risk of disease. By its very nature, sex requires trust. When you get close to someone you are literally putting your body in their hands, so you need to feel safe both physically and emotionally.

Take some time to think about where you and your friend fall in this spectrum of sexual comfort and what, if anything, you're ready for--in the end, you and you alone have to be comfortable with your decision.