Not exactly trying to defend the GF, but everybody keeps saying "cheating cheating cheating cheating cheating".
At least to me, a bit of sexting, and full blown fucking, are on two extremely different levels in the cheating hierarchy. As far as we know only the former has taken place.
Two different things, sure, but at its essence it's a serious breach of trust. Even if it was a one time thing and they stay together, OP will have to live his entire life second guessing her actions and wondering if she's talking to other guys.
You wouldn't want to live in a house with a broken foundation.