They either lose money when outside companies can't support the BSA's anti-gay views, or they'll lose money if and when chartered organizations (Latter-day Saints, Methodist Church, etc) decide to leave because they they can't support the BSA decision to allow gays.
To me, based on everything I've read here and elsewhere, this appears to boil down to money. With that being said, the way I see it, either the pro-gays are going to force them to lose money by way of pressuring outside donors (UPS, for example) to stop funding the BSA, or the anti-gays are going to bring in additional money to offset the lost revenue due to the lack of outside donors. The money is going to have to come from somewhere. Now it's just a matter of which side wants it more. You can't turn back time, and the pro-gay side isn't going to simply disappear, so the anti-gay side better start raking in the dough. Sadly, organizations like the BSA can't sustain on morals alone.