@Sarah Sarah ,
I wouldn't worry about the porn so much as the swinger sites and sites with dateable women. After all, there is an overabundance of porn on the internet. If he's taking to dating sites to find porn because he can't find it on the worldwide web, then he's not doing it right.
So you're left with some conclusions that aren't so very nice, including that he was at least considering meeting at least one of these women. Plus, personally, I'd be freaking a bit if my husband was searching for female friends' pictures as a porn substitute, which your boyfriend may (I say may; the jury's still out) have been doing.
Either way, though, you're clearly not trusting of him. And he seems to be proving and justifying your suspicions.
So, why stay?