My daughter and son in law both have opposite gender friends, granted my daughters good friend is gay but his friend isn't, she is straight woman with a boyfriend, they are childhood friends . It's never a problem for them. I think problem arises when it becomes secretive.

In situation with my daughter they all hang out and his female friend attended their wedding etc

My fiancée also has few female friends ( he has largely female type of profession and a hobby that is mostly female type so it's natural he knows plenty of women ) but I know them, it's not secretive plus they are all married and have kids. I am not concerned whatsoever.

Have you met each other friends? Do you do things all together on occasion ? If not, I'd say it has element of secrecy and I wouldn't go for it. If you BF has never even met that woman I don't even know what kind of friends are they? Cybersex friends?

I wouldn't tolerate secret friends but if it's real life friends and you both know them then it's totally fine.

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