I think it is sometimes the insecurities of both, actually. For the man, it's as stated: a woman couldn't possible be a "threat" to him because..no penis.

And, for the female, she gets to determine who he dates/has sex with, and so gets to keep "control," whereas, if he dated other women on his own, the female partner may think he'll leave her. I've never really seen unicorn hunters without veto power, and IMHO it's almost always the female partner that exercises it, and often/usually because of insecurities.
I also agree with Kevin about protecting the "sanctity" of the marriage. Read as: one penis policy. It's not really sex if there's not a dick involved, right? So, that's just not the same thing as if a dude was allowed into the relationship. Plus, the guy (generally) doesn't want to have sex with another man, so what's in it for him? /sarcasm
In theory, yes, financial stability is one benefit for the "unicorn." That assumes an awful lot, though, and can get into some territory that is, at least from my perspective, a bit...icky. But, for some bi-woman, it may be something they want or value. Emotional stability is something that is often touted by a couple seeking a HBB, but it seems to me that it generally isn't emotionally stable for the HBB, so much as for the couple. There's a lot of emotional wreckage when a FFM triad doesn't work out, and the "third" almost always get, by far, the worst of it, esp. since there's usually an expectation of poly-fi. Becoming financially and emotionally entangled in a situation where two people have what amounts to control over one, which is often what happens, is a real possibility for "unicorns;" so, while I think the couple sees the financial and emotional "stability" as things they can offer, from a unicorn's perspective, I think it's really a double-edged sword.
Interesting to note that it is the man's (husband's) insecurities, rather than any the woman (wife) may have, that are protected in a unicorn-hunting situation.
I'm also having a thought that, in particular, if the original couple is married, that they aren't just trying to protect themselves and their relationship; they're trying to protect the sanctity of their marriage, and the institution of marriage as well. Traditional marriages come with vows/expectations of exclusivity; many people probably fear that their marriage will die if some vestige of that exclusivity isn't maintained.
Re (from
GreenAcres):
Right ... ahem, rephrase: We need the input of couples who seek an ideal MFF triad relationship.