Host: “I’m going to be having my (male) cousin over. Is that okay with you?
Me: “Yeah, no problem.”
Okay, I hear where she’s coming from. Five years ago I would have been a little leery of a Mixed Meal. But 6 years of dating later, I’m not sure what the point of avoiding men is. I’ve been out with loads of them, and the only negative fallout was boredom.
There are three possible outcomes to a mixed meal that I can envision:
(1) You don’t like the male at all, and cheerfully part ways. I’m thinking of that truly obnoxious guy who interrupted me literally every single time I opened my mouth, loudly inserting his own comment over mine. Zero exaggeration. I think I completed one sentence the entire meal. I found the psychology of the situation very interesting. Completely marginalized (thanks for nothing, host), I wondered if I should stick to looking pretty and commenting on kittens. Instead, I went off to help the teenage daughter pick an outfit for next week’s shabbaton.
Was it frumkeit? Did he not want to listen to a woman speak? Was he showing off? Trying to impress me with his knowledge? Or was he just a jerk? Who knows. I have no plans to get to know him well enough to ask.
(2) You enjoy their company, but just not like that. A friend of mine was at a few mixed meals (With married people supervision! You know, to keep the singles from misbehaving) with the same guy. They enjoyed exchanging ideas, so someone tried to set them up. Quothe the guy: “I don’t think of her that way.”
So it’s not just me who is perfectly capable of thinking about members of the opposite gender platonically. Wait: I know loads of men can, because that was their reason for breaking up. I’m not advocating air hugs all around, but I’m not convinced that sitting at the same table as an unmarried member of the opposite sex is going to tarnish anyone’s soul.
(3) You really like them and want to get to know them better. Whereupon you ring up the host, and someone gets to play shadchan. And what’s wrong with that? As DiT puts it:
“…Back when I was in the Heights, if a meal had a guy in it, I wouldn’t go. Meeting men when you’re trying to get married? Bad idea! What was I thinking?”
Bad for Shidduchim 
Hm. Good point.
early days in the heights- made every meal mixed, striving for that ultimate gender balance and making sure the guys weren’t complete losers, thinking maybe there was a chance.
later on- realized the guys were just freeloaders who either tried to cook and were miserable at it (points for trying, though), or brought moldy fruit (true story!) and packaged kugel because they didn’t want to host. and their company wasn’t enjoyable and that of my friends was. so i stuck to inviting my friends (including marrieds).
I am not aware of any Halachic prohibition against mixed meals, even though Chassidim customarily avoid them.
What is generally dismaying is your point at the end of #2. I often wonder how many people are negatively impacted because they think it’s not OK to eat with members of the opposite sex, when it’s quite possible that they could meet someone they actually might like. By the time people come around to realizing it’s not a bad thing, a lot of guys who might have been good fits are gone, and they’re left with guys like the ones with GP refers to.