I think it's a target audience mismatch, actually.
I don't know what PETA's demographics are, but when I imagine a PETA supporter, I imagine a vegan, probably female, probably militant feminist in her spare time.
By comparison, the target audience for visual pornography tends to be men (whereas women tend to be more into romance novels). And militant feminists if they're more second wave-style feminism, tend to be pretty strongly anti-pornography (there's definitely a branch of feminism that claims pornography is always degrading to women--it's a branch that has largely been outdated by third wave feminism, but PETA supporters don't strike me as the types to take a half-way opinion on an issue).
So...yeah, they probably will alienate a section of their existing audience with porn. Maybe they'll pick up a different crowd of supporters to make up for that?