I honestly don't know if there's really a reason behind it. I'm bisexual but currently married to a woman. When I dated (I dated quiet a bit back in college - both male and female partners) I just didn't like sex with men as much because semen just grosses me out. There's something about the feel, smell, and just look of it. I had a strict nothing in my mouth rule so I didn't taste it, but I imagine I'd be just as grossed out, if not worse. Nothing bad sexual or otherwise has happened to me growing up. I pretty much grew up a boring middle class life. I just honestly am grossed out by it period. Maybe there's no reason behind your dislike for it other than it just grosses you out.

Just my two cents...