Tl:dw; aids/hiv and assholes with agendas writing magazines.
I'll share some of my perspective. It may also include a little bit of jealousy from the queer side; a bi man can hide his bisexuality while a gay man cannot. It may also have a little bit of its roots in insecurity. I've been able to find men much easier than women, so the usual limits on a hetero man with regards to finding someone to cheat on their boo with don't apply to a bisexual man. An insecure woman could see that and want to avoid bisexual men altogether due to personal insecurity in her ability to catch and keep a man.
I don't agree with these perspectives but people are human, and those are very human emotions to be feeling.