If you feel like you need to ask someone if your BF is cheating on you then that would mean you dont trust your BF. Relationships are built on trust. I have tried to have a relationship with a girl I didnt trust and it didnt work.
Some guys ask a girl to be their girlfriend just to get into bed with them, tell all their friends a different story and just feed you with what you want to hear just to keep you sweet. Sorry if this scares you.
You should talk with your BF and let him know how you feel and why you feel that way. If he has nothing to hide im sure he will comfort you and reassure you.
Im sure you will be able to see in his face if he is truthful or not and if you have any douts then maybe its time to move on. Do you really want to live the rest of your life with a guy you dont trust, always fearing he may be doing the dirty?
I know I wouldnt.