What is it exactly that makes it wrong, without bringing personal faith or beliefs into it?

So like I think that mindset itself is kind of wrong. The problem is you are looking for some kind of objective moral base and I don't think one exists. Ultimately personal faith or beliefs will come into play. And when you go off of that it's usually some psychopath trying to justify their actions which makes people very wary of even having the discussion.

Like we can point out the obvious problems with zoophilia, it's animal abuse, and spreads disease. But that only works if both of us share the personal beliefs that abusing animals is bad, and disease is bad. Which I should hope we share, but for the purpose of discussion I'll become the devil's advocate here.

Can you find an objective moral base by which you can say animal abuse is wrong? What if I hold a personal belief that animal abuse is great. I love kicking puppies in the face! And I also think disease is wonderful, I try to spread it everywhere.

And I'll objectively defend both of these beliefs through Darwinian evolutionary pressures. Weak puppies need to be kicked into the face to cull the puppy population and produce stronger puppies that won't suffer, same with disease it culls the weak and makes the human race stronger.

I think this comes down to the main problem of what happens when you start messing with the ethics of society and changing social definitions. No holds barred really hard to self arrest from it. Why despite agreeing that euthanasia can be morally acceptable in extreme circumstances, I'm more or less totally opposed to the application or it in practice, because that idea that it's ok for doctors to kill people can easily spread to less extreme circumstances. We've seen it happen on a large scale before and it can lead to literal genocide. Like not silly internet hyperbolic nonsense. But actual 20th century Nazi concentration camps where they were going through and culling all the weak and disabled and otherwise undesirable elements of their societies along with the Jews and Gypsies.

How it relates to marriage laws, I think the general liberalization of sexual immorality laws is a good thing. No reason to arrest gay people for doing it who aren't otherwise causing problems. No reason to ban cohabitation of gay people. And I'm pretty much fine with them getting the same kind of benefits that married couples can get. Adoption is a bit tricker but I don't think gay couples are automatically going to be bad for the kids, but that needs to be evaluated carefully on a case by case basis to avoid problems like that one US case recently where the gay couple was essentially pimping out their adopted kid.

The issue comes from the redefinition of the social institution of marriage I think. You have to be quite careful with that sort of thing. The generalized attitudes of "love is love" don't really work because well, as you say if I want to marry 5 women and my horse as well, why not? It's true love!!! If you don't say so aren't you a bigot?