This topic is very interesting, and I have to say relevant to me and my relationship with my husband. We both do BDSM, and we do it together, and sometimes even with each other, but he is not my Dom and I am not his sub or the other way around. I can't imagine a BDSM relationship without sex, it'd be like having broccoli for dessert or something. I also love my husband very much and would never consider having a vanilla affair outside of marriage, and would be very hurt if he did. Somehow it doesn't seem the same in a BDSM setting. I don't know why that should be, but extra-marital BDSM sex just doesn't seem like cheating, especially when your partner has full knowledge of it and even may be watching.