Bisexuality

So I was listening to the Savage Love podcast today and an interesting question came up.

A caller told Dan that he came out to his boyfriend as a bisexual after an 8-year relationship, and his boyfriend freaked out. Over the past two years (totaling a 10 year relationship), his boyfriend has basically totally pulled away from him, and not touched him or "done anything" with him since he came out.

My first reaction to this was astonishment. I see it as incredulous that a gay man who has been in a relationship with another man for 8 years would react so negatively to an admission of bisexuality. I understand (as Dan pointed out) that the caller had basically been lying to the boyfriend for those 8 years (though that my be speculation - we don't know for sure if he ever nailed down his sexuality as being 100% gay), but at the same time, I don't understand why it would matter.

The caller then said he had waited to tell him because he knew that, with gay guys, bisexuality tended to be a deal breaker.

WHAT?!

I've never heard of this before. It may just be because I have tended to end up with bi guys, but... REALLY? There's a big occurrence of gay guys who refuse to date and/or are disgusted by bisexual guys? When Dan responded, he didn't act nearly as surprised as I was, and... that stunned me. I couldn't wrap my head around it. Gay people are supposed to be more tolerant of non-"normal" lifestyles, right?

I don't understand this at all. Any insight from my few readers?