While I'm sure that's true for many of them, I think this ignores the (probably substantial) pool of sx-doms who are not seeking a soul mate, and who indeed may not even believe in such an idea. I think that's the area @OrangeAppled
was getting at, and I would guess that it's just as common as the variety who need that perfect other person to "complete" them.
I think many sx-doms have a less idealistic approach, one that enables them to attract, attach, and let go more rapidly than might be expected of a hopeless romantic.
I tend to feel very anxious when a partner tells me he's "comfortable" in his relationship with me. I know that it's intended to be an expression of contentment, but for me, it's also foreshadowing the inevitable decay of attraction. Maybe that's my 7 talking, but I don't exactly feel compelled to stick it out for the long haul if we've already reached the apex of our excitement. In other words, I tend to fixate on energy, not so much on bonding (which I don't worry about, since I assume it will happen organically if the energy persists).
One could make the argument that the self-preservation in an sx/sp would foster a desire for the safety and comfort of, say, married life with the One, but I think it could also function in reverse. It's really about looking out for yourself first, controlling your own personal security. If you're more of a romantic, then sure, it might drive you to seek a soul mate. However, I'm sure there are plenty of sx/sp individuals who are just fine getting off and getting out, because it keeps them safe in a very different way.