As shkoyach said earlier there is a difference between an issur and a siyag. There is no issur for men and women to speak to each other. If there was then as jphone pointed out, how can a man go to a bakery and ask a women behind the counter for a loaf of bread. Chazal say to stay far away from arayos. There are some obvious issurim that can come about when men and women interact and therefore certain interactions themselves are assur. When it comes to arayos, in addition to the issurim that the Torah says bifeirush, there is an issur diyoraisa of “lo sikrivu”. Chazal explain this to mean that it is assur to do something that might lead to the actual issur mentioned in the pasuk. For this reason negia is assur diyoraisa. I assume that if a women is walking down a crowded street and a man brushes against her neither did anything wrong as long as it was completely unintentional (If one of them caused the accident then he/she obviously committed an issur and the other one did not). Speaking is a little more lenient in that a man is obviously allowed to buy bread from a woman at the bakery and speak to her in the process. If a man and women pass on the street on Shabbos they are allowed to say “Good Shabbos”. This doesn’t mean that they can have a whole conversation. There is no exact way of explaining what is assur and what isn’t. Every situation is different and you have to use seichel to figure out what might lead to an unhealthy relationship, but it is better to err on the side of caution and remember that chazal say to stay far, far away from arayos.
How do you apply this to online conversations? When men and women speak to each other online in a forum such as this, neither knows who the other is or what the other looks like. You can’t even know for sure that Joseph is a man or Bais Yaakov maydel is a women no matter how much they insist that that is the case. Even if you can determine another poster’s gender, you still don’t know their age and whether they are married. We have rules here in the Coffee Room about revealing personal information and therefore the issur of “lo sikrivu” should not apply at all because the chances of the conversations in the Coffee Room leading to actual contact between posters and then leading to issurim is very low. If, by some chance you do know another poster’s real identity and you are of the opposite gender than you probably should not be conversing with them online.
There is another issur besides “lo sikrivu” which is for men to have bad hirhurim (I assume that women acting untznius is an issur of lifnei iver regarding this). The moderators try to not approve any posts that may cause this. Of course, each person has to know himself and if having an online conversation with an anonymous women you have never met will cause this then you shouldn’t be having those conversations.