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.

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