a slight problem

Hi I am a guy and I am never sure about my sexual orientation. There was a point in my life which i think I was gay then I thought I was straight and now I am not sure whether I prefer guys or girls. I have had relationships with both guys and girls before. Now I am in univeristy but I am still not sure. Well sometimes I think I may actually be bisexual but this simple thought actually disgusts me. I just can't accept the fact that I am anything else (bi, gay) other than straight. This is a intrinsic concept that has been in my mind since I was a kid and with family/peer/religious influence so please don't ask me to change my mind on this (coz i have tried very hard to convince myself before and failed). Yes and i do regret what I did with guys before.

Now to make matter worse is that I am now living in a flat with two other guys (my best friend and other guy from the uni). I think I kind of have a crush at him. we do make gay comments to each other from time to time but I know perfectly well that he is straight and he thinks I am straight too but the fact is i aint sure myself either. I am confused and I just don't know what to do. Obviously this friendship is more important than you know, coming out and ruins our relationship.

So what should I do? and am I bi?