I don't think any business is "recession proof", but I think that the effects would be less harmful to our industry than many other luxury-type industries. It's not like we're heading into another great depression. Having a lexus is nice, but its not a primal need. No one pleasures themselves to rolex watches and mansions (as far as I know, maybe that a new niche...). As long as chicks keep being hot and guys keep enjoying looking at them, there will always be a market for porn.
Would an economic downturn cut into profit? Surely.
Is it ever going to be bad enough that its not worth it? Highly unlikely.
Just my 2 cents.