It doesn't extend into the rest of the relationship. Sadomasochistic pornography has no relationship to the sadomasochistic society we live in. If it feels good, go with it. Reenacting abuse heals abuse. Sadomasochism heals emotional wounds from childhood sexual assault.
Sadomasochism is political dissent.
Although formulated by its current advocates as an issue of sexual liberation, minority rights, or even healing, I consider lesbian sadomasochism to be primarily an issue of feminist ethics. I believe that lesbians who embrace sadomasochism either theoretically or in practice, are supporting the lifeblood of patriarchy. This is the big lie. Part of the reason that we are vulnerable to this lie is that many of us were raised with religious notions that punishment is love and that suffering is redemption. As women we are taught that love is selfless devotion regardless of the pain suffered.