The ASPCA and other related organizations were around long before gay marriage became even close to being accepted. (1866, if you’re interested, in the case of ASPCA.)
I don’t think there’s a correlation at all between bestial (yes, that’s how it’s spelled) relationships and the ASPCA. That doesn’t even make sense. Does that mean people shouldn’t be allowed to have pets? Or, from a different tack, what about people who donate money to orphanages? (Fill in the blanks here.)
You have no way of knowing to whom people will donate if there is no ASPCA. For all you know, they may not donate money to any charity at all. Maybe they will, but YOU HAVE NO WAY OF KNOWING. Your argument is the same one that people use to try to stop wealthy people from making big simchas and having big houses, but do you KNOW that they’d be donating all the money that they’d otherwise spend on them to tzedakah? No, you don’t. Don’t get mixed up in yenem’s cheshbon. We all have enough trouble with our own.