I don't get that, seriously. To me being intimate is being intimate. You were infuriated when she confessed to having thoughts about a MAN (look, his age, his looks, really arbitrary for the sake of this discussion), yet you encouraged her to pursue an encounter with a WOMAN. What's the difference? She's being intimate with another person, that's what it boils down to, male, female, it's another person and you're not involved.
A threesome is one thing, everyone is onboard (I can kind of understand this, especially if it's a healthy long term relationship/marriage), but saying sure, you can go sleep with her ... but being pissed about her having the hots for another man?
Explain why one is okay and the other isn't, help me here.