Doesn't sound wierd to me.
That's probably because you studied or were taught grammar?
I think that most people naturally 'feel' that the word couple has a plural assumption.
So we wouldn’t say;
‘The boats is two in number’, we’d say, ‘The boats are two in number.
Or,
‘his testicles is swollen’, we’d say, ‘his testicles are swollen.’
Or,
‘Summer and winter is my favourite seasons’, we’d say, ‘summer and winter are my favourite seasons.’
And therefore we say; 'The couple are in their sixties,’ rather than, 'The couple is in their sixties.
So, turn of phrase speech patterns triumph over grammar all of the time. We use the language as it suits and so it evolves. Grammar is only ever playing catch up.
Except for this man (the final speaker)
https://www.youtube.com/watch?v=QereR0CViMY