No it's the reality of sexual relations. Marriages have ups and downs. For one having sex with a lot of partners makes it less special. It becomes an act, not an expression of love and a special kind of closeness. This psychological effect is present in both multiple partners and masturbation to porn. Degeneracy is a problem, whether sexual, drugs, or general immorality. I'm not insecure in the least, I've had plenty of partners and am above average in bed. As I've matured I believe having too many partners has affected how I approach sex, and I've definitely seen this in my wife's views of sex. The stated number of 10 has been written about, I wouldn't say it's a hard and fast rule as much as a guideline to what you get when you go far over it.
See, here is where you kind of take a turn for the puritan (and also sneak in a humble brag). I would contend that building a marriage with sex as one of its primary pillars is doomed to failure, no matter HOW many dicks she rode. I actually think Khane has the right idea here, at the basic level. Numbers is not the issue, its a self esteem thing. If one partner feels insecure and either of them make sex a central feature of a relationship (rather than a nice perk), shit is going to go south no matter who was racking up the numbers. Thing is, people in general, are very insecure about relationships, at least when they are actually emotionally invested in someone. So from a practical standpoint, its more about what the right amount of dicking results in not being hung up about sex or feeling a lack of security in your relationships. As Khane has pointed out, this has changed over time and has purely to do with social mores, not some magical threshold.
As a society, we are pretty open about sex in the modern era. This can make people insecure because they know if they can't do their partner right, there are lots of other options out there. And then you have dudes who simply are tired of having sex weaponized against them who decide to just rub one out or hire a bang maid, to avoid the whole dance. People who have explored options in the past are more inclined to say "fuck putting up with this minor inconvenience" and move on, thanks to society largely being much more accepting of it. Divorce has basically been normalized and practically expected in places like the major cities. So between lessened social expectations and also the end of the single income household have increased both the ability and incentives for giving up on a relationship (or even having one in the first place). And in Khanes numbers a lot of those 2-3 partner high divorce rate people are the cheaters of that era and the couple of extra people are the ones they fucked around with, thus ending their relationship.
So why is 10 the magic number now? Well, think about how easy it is to get laid nowadays and remember the pussy slayers back in the old days. They would move on at the drop of a hat (as would extremely attractive women, in certain cases) because they could. The sub 10 dicking crowd are basically people who mostly are so flawed or insecure that they don't want to risk trying to find something better or different. Or they were brought up in the old school of thought on relationships and marriage (i.e. Mormons) and simply prioritize things other than sex for their relationships. Short version is when the midlife crisis hits the current generation, their experiences from banging like rabbits in college tell them they can get that need taken care of pretty easily and its one consideration for keeping a relationship together or healthy.
Want some anecdotal evidence, then look at recent events in the Divorce Thread. Wormie had a woman paying for him to go back to school that also did anything a man could ask for and he STILL traded her in for a younger version of the same thing.... because he could. Cut's woman pulled the ripcord the moment she got her first and only other dicking. Basically, alternatives that would be unthinkable, socially, are out there now and the ten plus partner club have the knowledge and confidence to take advantage of that, without a lot of the shame that once accompanied that. In the past, the person who sowed their oats locked shit down once they found something they liked, because there were social obstacles to doing otherwise.
Not to bash you here, but you pretty much sound like the church preaching on this point. I agree with your threshold (numbers do not lie), but not your reasoning, Titan. For once, Khane is the voice of experience here and has the direct insight.