Well, it is a matter of faith for me. I was confused about whether free-mixing was haraam or not when I first started practicing. I actually never used to free-mix before I started practicing and I used to only have female friends. But after about a year of practicing I started that. But it never really felt right to me. The brother was practicing too, and i suspect he knew it was wrong too. But anyway I was really confused and I made du'a to Allah to let me see what was right. And a couple of days later I got my answer. So personally I know what the right answer is for me. Since then I have read up on the matter and I feel that I definitely made the right decision.