Sex and coworker

My coworker want to have sex with me I have a bf and he have a gf/fiancee he keep telling me he want to have sex with me we flirt a lot too I love my bf but me and my bf have sex once or twice a month and it don't last long but my coworker have sex with his gf whenever he want more then me and my bf do I'm tempted to have sex with him I have talked to my bf numerous times about us having sex more but he always say he be tired from working different shift what should I do?