Ehh, sure the acts get worse and worse, and soon it's just not enough. But they already have screws loose, and it just cascades downhill from there. I dont think BDSM has a goal actually, and I'd also like to continue to point out the difference between love and sex, or forms of sexual gratification like BDSM. It's not the same. I tie the two together in the marital bedroom, and if everybody is happy and still in love, it's ok. Cross lines that the other is not ok with, and you jeopardize the marriage, and thus the sex being ok. I think many married couples are secure enough in their relationship to explore stuff like that safely and keep the spark hot! But it's got to be mutual, I cannot imagine a scenario where my wife would ask me to do something I was not comfortable with or vice versa.