More than a few submissives enter The Cage because a vanilla Significant Other emotionally cannot, or simply will not, become the Dominant they have come to crave to satisfy suppressed needs. Fair enough.
Finding a totally discreet trustworthy Dom who accepts the sub's entanglement is hard. But in many cases an even harder situation can ensue. We'll take it as a given that in ways other than sexual and emotional satisfaction, the sub loves her (or his) SO and does not want to end it. But the new and powerful D/s relationship can soon lead to a growing second love. This duality can eat at the sub even as her raw craving to submit threatens to destabilize her primary committed relationship.
Understanding the danger, finding the way to balance these two loves in her life, can become her thicket of emotional thorns.
A Dom who takes on an 'attached' sub would do well to be especially sensitive to her dilemma should he sense love developing out of his dominance. Having a marriage, or any attachment, and then being overwhelmed with loving thoughts toward a Dominant entering her life can be an emotional tsunami for her.
With an attached sub, we might ought to tread cautiously around this potential relationship terminator.
My own track record has not been as aware as it should have been.