Sitting Cross-Legged Harms Your Knees?

I have always been sitting cross-legged, I can sit for long hours because this is always been my habit. I am always comfortable in this position. I also appreciate the Japanese way of sitting. I believe Your legs become stronger by either sitting cross legged or the Japanese way (Vajrasan)

However recently I came to know that sitting cross-legged is bad for knees. It also causes trouble of Varicose Veins. Is it right? If yes, then what is correct posture of sitting anyway?