Bisexual women get annoyed when they have straight male partners who want the rules of their relationship such that the man can date whoever he wants, but the woman can only date other women.

I understand that, and it does seem unfair, although some couples have reasons for having unequal arrangements, or use them as a stepping stone. (Links possibly NSFW.)

However, it bothers me that a type of consensual relationship is disparaged as wrong in general, when it's something that the people involved agree to and find workable.

All that said, I'm a little confused, because the comment that you responded to was about dating multiple bisexual women, not about a one-penis policy. The comment was:

If I could get into a relationship with a harem of hot bisexual chicks, I'd do so.

I read this as either referring to a closed, polyfidelity relationship, where everyone is limited to sex with people in the relationship, or as completely open, where any of them can have sex with any other people (male or female). Is there a one-penis policy implication in there that I'm missing?

Also, does the one-penis policy objection apply to polyfidelity where there is only one male in the relationship? That situation seems more equal.