Unfortunately that is, in my opinion, a wrong conception. I know guys who have no problem with my sexuality, but who are very much turned off by a man doing anything to them. I also know that there are a lot of "true" bi guys out there.
Now before anyone starts to blame me for trying to "Hide" my sexuality behind being "bi", I first came out as "Gay". That being said, I had always wondered why I loved the feeling of fucking women, eating their cunts out, and trying to drive them wild. And yet, if I look at straight porn, it is still the men that get an erection out of me, before most of the women. So, I'm more gay, than straight, but I could enjoy sex with either sex. So I finally had to face the fact, I wasn't fully gay, like some of my friends who seem to pass out at the sight of a woman's vagina. I was actually bi-sexual.
I also know a few guys who have played with me in the past, but when given the chance to play with me, or be with a woman, they jumped on the woman first. They'd tell me, "Thanks for the offer bud, but I have this hot chick I want to fuck." Now if they'd prefer to be with a man, than why did they pass up on what they loved to get, when there wasn't a woman around?
I do believe there's some men who do hide behind being "Straight", but willing to let a guy suck their cock. I won't deny this fact. But there are also many examples of "Bi" guys out there, that I've met. We do exist, and in different flavors, from mostly straight, but like a little gay sex, to mostly gay, but enjoy a little straight sex (as in my case). We're NOT all trying to hide from being "gay".