I met my husband in December 1974 and we have been together since January 1975. We rarely have a cross word and still enjoy each other's company very much, but that doesn't mean that we don't enjoy other people's company, too.
Sometimes, my interests coincide more with those of my friends than with those of my husband. He loves sport; I love writing, photography and history.
If I want to go to a history event, then he might suggest that I go with one of the historians I am friendly with, who happen to be male. No red flags required, it's just common sense to go with an interested friend. My husband doesn't mind, because he trusts me and thinks it makes sense. But I don't.
I might go out for the day to a castle, taking photos, with a friend. If it is a female friend, no-one thinks anything of it. If it is a male friend ~ well, it doesn't happen, because it doesn't feel right, unless it is as part of a big mixed group.
It is a bit of a shame that we are not 'allowed' to have friends of a different gender ~ but I do, of course, understand why.