On another occasion, one prospective date took one look at the writer/actor/director, frowned and declared, "No, you're too old for me. You look over 35."
"Can you believe that level of cruelty?" says McKnight, offering a wry smile. "Amazingly, in situations like this people feel they don't need to spare your feelings. They can be really brutal."
The "situations" McKnight is talking about emerging from his experiences with gay relationships website Grinder.
"Basically, what happens is you have an app for your phone, onto which you can upload your photo and then chat to singles who are in your area," he explains. "You then meet for a coffee or whatever.
"But it's problematic. I once turned up to meet a guy at a bar, on a Saturday night in Glasgow, and then asked him what he wanted to drink. Before I could say 'Two Coronas, please!' to the barman, I turned around and my date for the evening was gone. There I was at the table with nothing but two beers and a miserable night in front of me."
The adventures on Grinder were not entirely wasted. "I figured Grinder dating could make for a great device for a rom-com. Then, when I heard a straight version, Tinder, had been released, I saw how dark comedy moments could make for really interesting theatre."
McKnight always makes interesting theatre. He's the Heston Blumenthal of Scottish theatre, evidenced by his work with Random Accomplice, and plays such as Love Hurts and Small Town - Ardrossan. He loves to challenge perceptions, to shock, but always to create laughs. His new play, Wendy Hoose, underlines all that is Johnny McKnight.
"I wanted to write this play about what it would be like for a bloke to turn up at a girl's house.
"I wanted to write about expectations, and perceptions we form so quickly of the partner for the evening. But I didn't know what to do with the idea."
The press blurb for the show reveals "Laura and Jake just want sex. Late Friday night drunken sex. Nothing more. No strings attached."
Jake, we learn, is a standard, no nonsense Glaswegian guy out for a good time, who says what he thinks. And Laura, well, she also wants uncomplicated sex. So what's the McKnight twist in the tale, Johnny?
"When I was working on the stage show of The Wickerman, I met a young actress, Amy Connachan," he explains in unusually cautious voice.
"She gave me an idea. But I wasn't sure how to progress it until I met with Robert [Softley Gale], the director behind Birds Of Paradise Theatre Company [which promotes the profile of disabled artists]. We agreed to co-direct and Robert was right up for my idea. He said he didn't want to do a play that was worthy. He wanted to do something funny, and that was exactly what I set out to do."
To reveal the premise McKnight came up with would really spoil the impact of the show, but it is safe to say the storyline is surprising and shocking as well as funny. James Young and Amy Conachan play the internet dating couple with just the right notes of curiosity, indignation and feeling.
"This is a romantic comedy pushed to the extremes," says McKnight, grinning. "What I wanted was the characters to be as un-PC as possible, to speak as they would speak in a heightened situation. Robert and the cast agreed, and that's good because there is some quite filthy sex in this play. I know my maw would be dying on the spot if she saw this. But the thing is, it's funny.
"It's not Chekhov, but it's real, and it's not patronising."
It's hard not to be intrigued by McKnight's challenge of creating a rom-com that's sexy, shocking and full of black humour.
"The thing about meeting someone for the first time is it can be all of these things. Internet dating can be really strange. For example, some guys' profiles actually say they're looking for a man who's 'straight acting-straight looking.' That just escapes me.
"But - as is the case in this play - I've also had that experience of turning up at someone's house for a date and being rejected out of hand."
McKnight breaks into a laugh. "One bloke actually said to me, 'You're too camp!' But this out-and-out rejection seems to be considered fair because you've exposed yourself to an app. How that works I don't know: but it does offer great scope for a play."
Wendy Hoose runs at the Tron Theatre, Glasgow, March 7-15