I need some advice.

Hi i'm 18 and don't know how to handle these thoughts. I think about have sex with a guy taking and receiving each other and orally like it gets really freaky. But i've been straight my whole life. I have a gf of 3 years and she is great. I love having sex with her and just being with her. It's just these thoughts have been happening more often and i really want to act on them but morally i believe that no one should cheat on their SO. I've tried self prostate stimulation w/ fingers which didn't help at all just left me yearning for more and i couldn't really get to the prostate. SO absolutely wont peg or finger cause she just not into that justifiably.Any tips/advice would be appreciated THANKS!