>Some studies I read about - especially the Kinsey report - mention something around to 50% for this particular habit.

That would be much closer to the reality, IMHO. And, for most of them, I would say it relies on long term relationship. That is one statistic that is usually not reflected when they release such numbers. It is easy to say things like that for long term relationships and boost up the numbers. But, if you would take, for example, a 3-5 year relationship, I would say the numbers are much lower.