Personally, i really don't like IM with a total stranger. Instant messages are like superficial chatting ("How are you? What are you doing?" etc.) and i can do such light chatting only with a person i already know a little bit more. That's why i even disabled IM. However, i do reply to mails if they interest me, i.e. if the guy has a revealing profile with pics, text, and arouses my interest with the profile as well as with his mail message. if he interests me and is decent, polite, open, patient, he will probably get a reply and i would want us to find out if we can develop futher communication based on common intersts, trust, openes etc. (even going to IM later on). if the profile doesn't tell me much about him (and so doesn't help me to see if i am interested or not), and/or the message is just sth like "Hi!"/"Hi, cuttie.../I like your picture..." etc., he most probably won't get a reply....
Internet communication is not like real-life communication, but it can reveal a lot about the person, not only by what is said/written but also by what is not said/written (even on the profile). So, we have every right to judge the person by it, and to follow our own pattern of developing trust, communication, liking etc. Pushing, prejudice, big expectations and fixed ideas are usually deterrents....
I hope this helps you understand "unresponsive women" a bit more, but of course there are all sorts of people here, and thanks God for that...