Maybe you can have a talk with your boyfriend and discuss how you can be better satisfied sexually. It probably isn't helping either of you for you to suffer in silence.

Then maybe you wouldn't be thinking about his fraternity brother so much in waking life, although it is still OK to have dreams about his fraternity brother, of course.

Whatever the problem with your relationship with your boyfriend is, it's better to discuss and deal with it than try to solve it by being with someone else. That won't solve anything.