While porn sites might be riddled with malware it is unlikely that they'll be zero days. More likely you'll see exploits for bugs that are publicly known but not yet patched everywhere.

The value of a zero days is largely rooted in the fact that it hasn't been disclosed publicly and any widespread use of a zero day threatens that value. Zero days will be used when the risk of discovery is very low or the payoff is very high and attacking random people who visit dodgy websites is unlikely to meet those conditions.