JMO before anyone comes at me.
Going for a meal or a drink with members of the opposite sex (when you are in a hetero relationship) is fine if there is more than one. Going with just one person of the opposite sex, just the two of you, is a bit odd IMO.
I mean, you have to take numerous factors into account. ie; were you friends before you and your partner/spouse met?
Also does this other person acknowledge you?
DH had a friend at work - a woman 20-ish years younger (mid 30s, married - 1 child age 12,) who he would stay at work and chat to for half an hour when his shift finished and hers started. Twice a week he would do this, and he would come home and tell me the gossip she had shared.
She has left now (2 months ago) and he chats to her once every week to 10 days for about half hour on WhatsApp, (just a catch up) and that's fine IMO. I have spoken to her some 4 or 5 times, and met her 3 times, (2 of them at a social event.) She is friendly and charming and always asks DH how I am, and she added me on Facebook, and 'likes' my photos. (DH doesn't have Facebook.)
He wants to meet her for a coffee in a few weeks and I have been asked to come. But I don't know her really, so have said to DH that I will go with him, and look around the shops while he has a half hour to 45 minutes catch up with her, and join them for 20 minutes afterwards and have a coffee myself. They are work colleagues (well, they were for 3 years til she left) and I wouldn't have much to talk about to her.
So as far as I'm concerned this is fine.
Conversely, one woman used to work at DH's place (mid 40s, divorced, 2 DC aged early 20s,) was very touch feely with him, called herself his 'work wife' (vom!) and when he had Facebook too, I would @ him in on photos, (she had him on her friends list but not me.) And if there was say, 25 photos of one day trip (10 scenic ones, 5 of me, 5 of both of us, and 5 of him,) she would just 'like' the 5 photos of just him, and put silly 'totes hilaire' comments on them, completely ignoring me and the photos of him with me and all the scenic ones. Just responded to the pics with him on them only.
When we bumped into her in town, she would started laughing and joking with DH, and say stuff like 'we have a right laugh don't we Steve?' 😆She would shove him (gently) and said once 'some of them think there's something going don't they? hahahaha.' DH was like
'do they?'
She carried on LOLing and guffawing, and tapped the side of her nose and winked at him, like she and he were party to something that I wasn't, and secrets were being kept. DH was like WTF???
She pretty much ignored me whenever she saw me and made a beeline for DH.
Call me naive if you like but when he said 'I haven't got a fucking clue what she was on about' I believed him. I can tell when he's lying. Anyway, if he had said he was going to meet her and her alone, I would be like 'NO.' (And he would be the same if the shoe was on the other foot.) It's not a matter of trusting or not trusting him, I just couldn't bloody stand her, and she would probably have told people at work that 'something happened...' Horrible woman. I was glad when she left a few months later! She deleted DH off Facebook about 3 months after she left.
So yeah, tl;dr it depends.