I don't know that cycling causes bone loss, it's just that the more you ride, the less time you're probably devoting to impact activities.

I was just in the LBS the other day to buy some cables; while the parts guy got them, I talked with the (female) cashier a little bit about triathlon etc. The parts guy caught the tail end of the conversation and explained how he doesn't run any more and that "running is for people who don't have bikes." I replied, "Running is for people who don't want osteoporosis." You should've seen the look the cashier gave me.