I'd also like to point out the extreme wrongness of OP seeming so judgemental of his (likely now EX) gf "having another guy IN HER!!!!1!1!"

Like...no. This is not ok to treat women like our vaginas are PROPERTY. And SULLIED BY ALL OTHER PENISES BUT MIIINEEE!!!!!

This is not a thing. This is not an ok attitude. This is misogeny.

That being said...I 100% believe the gf went on a break as a selfish guilt-free way to sleep with that guy. It is dishonest and manipulative. It is wrong to do this to a partner and I would not tolerate it at all.

People often have different ideas about what "a break" entails. OP believed tey were still quasi-together and would remain monogamous through it. GF did not.