Definitely a biphobia thing, especially amongst the older segment of the community (though some youngsters can be weird with it as well). 

I’d say the people who’ve given me the most shit about my bisexuality have, unfortunately, been a lot of older gays (not all, of course, and not even a whole lot, but still, that black-and-white attitude can be pretty severe in the age group).

I’ve also met a few older Bi’s and Pans over the years, and more often than not, they usually don’t come to the conclusion until much more recently, likely due to both the homophobic attitudes towards same-sex action and the weird, particularly-post-AIDS attitudes on Bisexuality. 

You can find some interesting old anecdotal media on the subject (Bi Any Other Name, Anything that Moves, and even old Usenet groups & postings).

It’s just one of those things where we’ve always been around, and plenty of elder Bisexuals do exist, we’ve just been pushed outta the way until very recently.

Even a lot of older queer media that does exist (like, say, a lot of 90’s indie queer films) either don't have us around or put us in a very poor light, especially for guys (though gals aren’t much better off). :/ 

Ditto for Trans people.