Humans value both intellectual and physical attraction. As if that is not reasoning enough, there is the anonymity allowed online. I asked G why he chose the internet instead of real life. No one knows who I am. It is harmless fun.
Physical limitations become minimized and almost non-existent.
If there are real life relationships, then there is a heightened sense of danger that one or both parties may be caught by their significant others. In addition, geographical limitations are no longer important. The short relay time of correspondence between two parties negates the lack of true physical intimacy. Plus, participants can usually engage in kinky activities online that are taboo in real life. Last, it is usually safer than real life.