It's a very difficult one.
Because, in theory, we should be able to trust our partners and we should all be free to go out for a drink with whoever we like.
My exhusband and I never placed restrictions on each other and we both had a couple of opposite sex friends we met up with occasionally. Other than that, we were always at work or at home with a young family.
Before my ex husband drove, he had a hobby on the other side of town. He told me that one of the divorced women at work (a friend of his I was aware of but had never met) told him that he could crash at hers after the hobby rather than navigate the limited public transport home. I was ok with it but his mum gave me a very stern talking to about 'giving my husband away'. Anyway, he never took this woman up on her offer.
The following year, he had an affair with a different woman at work whose name he'd never even mentioned but, who it turned out, he'd been meeting up with for lunch. I still don't think he'd have cheated with the first colleague because, if he were going to, he would have.
And that the crux of it.
If someone wants to cheat, they will. No amount of 'always at home', 'doesn’t have the opportunity', or weekly lunch 'dates' will make any difference.
But, at the same time, affairs often start in the same way as primary relationships. They don't start in the bedroom. They start with conversations, developing a bond and spending time together. The only thing ever stopping a person from cheating is opportunity (ie mutual attraction) and personal integrity.
I'd never stop a partner from meeting up with an opposite sex friend but would also be aware that it could turn into an affair and that i have no control over that. But, then, they could have an affair with someone they'd never even mentioned. Or have someone throw themselves at them and still never stray.
Only they can make that decision. All we can do is choose someone we don't think/believe would cheat in the first place but, even then, there are no guarantees.