It's a potentially hot idea, but for my wife and me, the women have to think of it as making the man the sex object, not as "servants" who are doing his bidding without fully getting off on his hung status, as well as him getting off on it to the extreme. It has to be totally mutual or for us, the roles have to be somewhat reversed to where the male is being sort of "used" (but fully enjoying it).
If his big dick is SO desirable that the women agree they are happy to share him (because of the simple fact that there are statistically so few super-hung males, too few to go around on a one-to-one basis), that is HOT. It affirms the power of the big penis as a super-desirable characteristic.
It also reminds me of the "alpha males" in the animal kingdom, where the biggest male gets all the females in the group. The animal-ness of it, applied to people, is great fantasy material, really. It's almost like a benign form of bestiality, not really involving animals, but invoking our own animalistic nature as humans.
My wife loves the idea of more than one male partner at a time, and we have had 3ways with several well hung men over the years. I LOVED it, seeing how much pleasure she got out of it. It made me much more bi than I ever would have been if I had only had experiences with males myself, without her. The vicarious and voyeuristic aspects of a "harem" are part of the excitement. For those with bi fantasies, any harem, male or female, may be a fun way to accelerate them. You get to relate more to your opposite-sex partner if you see her or him with additional sex partners and accept their enjoyment of them.
There's at least one LPSG member whose gallery has pix of him in a hot tub with three other women, and they look like they are all having a very GOOD time. (I have to search for him..."bonanza65" maybe? Not sure I'm remembering it right.)