› firstname.lastname@example.org 
I wish I hadn't read your column about cervical penetration. I am thoroughly disgusted, disturbed, even angry. It's been my understanding that body modification is a red flag of imbalance, poor sense of self, and ignorance about anatomy and physiology. Do you think I'm justified in being disturbed that people are going around messing with irreplaceable tissue and getting off on their own pain? Throughout history we have tested our physical limits through sports, battle, fasting, etc. I can understand the desire to be liberated from our fear of pain by initiating it ourselves. It seems like a misunderstanding, however, to think that in our search for spiritual enlightenment, we will transcend the physical by destroying it. I'm hoping you can explain so I can stop feeling horrified.
Puzzled by Perversion
There are ways to indulge that are probably harmless, and there is stupidity (you ought to know better) and compulsion (you do know better but can't help yourself). Since the young lady had a desire that could not be pursued without causing herself injury, I gently steered her toward what I hoped would function as a harmless alternative. What I did not do was presume to know her motivations or assume that she was broken just because she wanted something I thought was ill-advised.
We should make a distinction between body modification — generally understood to mean piercing, tattooing, maybe some scarring, and the occasional loony tunes full-body job like that guy on the Discovery Channel who looks like a big doofy cartoon cat — and S-M. The pain usually but not always inflicted or received in pursuit of the latter may look like a mere ouchy sensation but can feel like anything from a massage to a form of worship, depending on who's doing what to whom. You can't tell just by looking, and certainly not if you use preconceived notions of what damaged people they are, doing such damaging things.
Body mod is usually done to, well, modify the body. You may think it's dumb — hell, sometimes I think it's dumb — but you don't know what motivated the pierced or tattooed person to mark themselves. Your Anthro 101 explanation actually supports my argument more than yours: "throughout history humans have tested our physical limits," indeed, by poking sticks through our noses, hacking bits off our genitals, and rubbing dirt into holes in our faces. While I'll grant that the people who invented these practices knew little of anatomy and physiology, I'll wager that they were neither unbalanced nor lacking a sense of self. They were (as our modern practitioners often still are) both seeking to set themselves apart from the other tribes (them) and be instantly identifiable to their own (us). This was more useful and important when the "them" tribes were wont to hit you with a rock and then eat you, but it's still an essential human urge, and decorating ourselves is a pretty harmless if occasionally silly way to express it. Does that help?
I'm in a long-distance relationship and having serious sexual frustration. I can't bring myself to masturbate more than once a month. I dream about masturbating but can't bring myself to actually do it. I feel dirty and wrong, and during my big one monthly moment I find the only place I'm comfortable is the bathtub. Is there something wrong with me? I was never like this before.
Masturbation, which for a long time appeared in popular culture mostly as joke fodder and the object of many undignified gerundial nicknames of the "choking," "draining," and "clubbing" variety, has been undergoing an image upgrade of late, in some circles anyway, often finding itself exalted as a holy experience or at least revered as therapeutic and educational. I'm down with respecting it — masturbation is useful, and you can learn a lot — but I'm never too big on the sacred. Nothing's sacred if everything can be made fun of, and everything can be made fun of.
All that said (and it was a lot), it seems to me that you're putting way too much pressure on one humble little act, imbuing it with too much meaning. If you used to masturbate regularly but not excessively and without guilt and that has suddenly changed, poor little masturbation is surely standing in for something else. I'd be tempted to suggest that you are depressed or developing an anxiety disorder. A change in attitude that dramatic about something that quotidian and harmless cannot mean anything good. If attempting to desensitize yourself by simply doing it more (and, we hope, thinking about it less) doesn't work, I'd get some short-term therapy, not because it's crazy not to want to masturbate (it most certainly is not) but because it's dysfunctional to punish yourself harshly for wanting to. Plus, come on, only OK in the bathtub? How Freudian can you get? That's like suddenly manifesting a fear of snakes or dreaming of trains going into tunnels. Something's up.