If she's just a stupid chick I don't see why not take advantage of her desire to be a cumbucket. If she's a full on retard, that would be different, but you seem sure she isn't.

If she's telling you she's a freak and up for anything, I mean, how can you feel guilt, it's obviously not her first day at the rodeo.

As for your girlfriend, yeah, what's the deal there, I don't think it's right to cheat, but maybe you and the gf can both take some pleasure in turning the quasi-retard into a human toilet.