stealthmongoose
Furthermore, on your note about the spiders; We don't typically rights of sentient personhood to things that don't have the complex neural network necessary for self-awareness or identity.
It's kind of why fetuses aren't considered citizens, among other things.
http://discovermagazine.com/2007/jan/cockroach-consciousness-neuron-similarity
Most people say, 'For heaven's sake, a bug isn't conscious.' But how do we know? We're not sure anymore. I don't kill bugs needlessly anymore."
Tiny Brains =/= Less Complex Brains.
We tend to be biased, as humans with our primate-based intellect. I assure you, that does not make our perceptions any better or more valid than any other non-human animal. It certainly does not guarantee our survival as a species, either.
http://www.utilitarian-essays.com/insect-pain.html
We (the general populace) don't tend to acknowledge that non-human animals are sentient, because then that would just make us look and feel bad.
It is a fallacy to compare a fetus to a fully fledged adult of the same or another species, especially <20wks.
Also, it is legal to have sex with animals in Denmark, as long as you don't abuse them. Animal sex tourism is becoming a thing there. And killing baby giraffes to piss everyone off.
Do I think you should be allowed to get legal benefits from marrying an animal? Hellll nooo. If that were the case, I would totally marry a spider! Marriages are stupid anyways.