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Part of BDSM (specifically the DS bit) refers to Dominance/submission. A couple's engagement with D/s runs on a continuum. So, some couples may involve themselves with it strictly "in the bedroom." For others they make it a 24/7 lifestyle.
There are several reasons that I identified her relationship as BDSM. For example, her calling her husband "master" is total BDSM stuff. Also, the fact that he chooses what she should wear for the day, etc. This is all part of a 24/7 BDSM relationship.
Clearly, the bible doesn't demand that couples establish this kind of a relationship in order to fulfill the recommendations for a biblical marriage. A man being the "head of the household" doesn't mean he makes every decision (nor, in most cases, is it practically possible).
The creation of this kind of relationship demands a lot of specific kind of work on the part of the people involved--work not necessary in a typical marriage. Essentially, it's the creation of a relationship that is rooted in power dynamics. Personally, I have my own feelings about the theological soundness of basing a relationship on power dynamics. I think that the bible encourages Christians to base their relationships on Christ and on being Christlike. This (to me) philosophically conflicts with the approach of BDSM. There are practitioners that identify as Christians and I don't know how they reconcile the biblical verses regarding humility and the "first shall be last" with their lifestyle choices...or if they even bother to examine the underlying philosophies to find any conflict.
However, it IS important that people know that this is NOT a reflection of what the bible demands of husbands and wives. It's possible that it's even a distortion of what the bible demands. While on the surface it uses some similar terms "submissive," "he is the head," "pleasing my husband," the relationship is not rooted in love or Christ, but in power dynamics. The people may love each other, the people may identify as Christians, but the ROOT of their relationship is power.