I can only speak for myself but..
My ex girlfriend, who I was with from 16-22 was apparently 100% straight, she would claim this before each "session" and again after it. She had a male partner who knew nothing of this and she told no one. She even lied about it to me, which given the circumstances was more than a little confusing.
However, I've known many straight guys who openly admit to having sexual relationships with men to various degrees.
I've lost male and female friends since coming out as bi. I honestly don't see it being a strictly gender based thing, it seems to be more down to the individual and the friends they keep. If you surround yourself with people you don't think will understand, you're less likely to say anything, male or female.
It takes a strong person to admit to not being what they've always claimed to be, no one ever knows how others will react and you need to be strong enough to risk losing everyone you care about.
But yes, there is a clear difference in most cases of guys and gals coming out. After all, we all know lesbians are hot and gay guys are only after converting the straights

(You know thats a joke right?)