they weren't always considered slutty. Its just that in the recent decade they have been increasingly popular with young women (who happen to dress more revealing or who engage in lots of sexual activity). After a while, when so many girls who are considered 'tramps' have them, its started to be considered a sign that someone was a tramp. Another reason is that when you have a lower back tattoo, you have to wear belly tops to see them, so if people know you have one you have to be 'showing it off' by wearing very revealing shirts. Not to mention people who have lower back tattoos tend to not get anything meaningful, they tend to go for things that seem a little 'slutty'