This always seems to be some guys' biggest hangup. They love the idea of opening their relationships, and visions of threesomes with another woman dance in their heads. And then the woman has to ruin it all by suggesting she might actually prefer a male playmate/partner.
We could speculate for pages on the sociological/psychological reasons why another penis poses such a threat for these men, but the point is such an attitude is rather shortsighted, and not particularly considerate of their female partners, who they are expecting not to have the same hangups about another vagina in their bed.
Personal: I was the one who went to my wife several years ago about the possibility of opening our marriage, because I'd found myself attracted to a polyamorous friend of ours and wanted to explore something more with her. In doing so, I didn't have to assume with my headstrong, bisexual wife - if she consented to it, it meant by default that I accepted her exploring the possibility of relationships with other people. And I accepted that that might well include someone who owned a penis. It wasn't a question of being "more evolved" or enlightened - just seemed like common sense.
After all, I would have laughed like a hyena had she come to me asking to open our marriage so she could date a guy, and I was free to date others too, so long as they weren't female.