I am a cyclist. When I'm training, I definitely don't recognise this:
If it's an all day thing there will be stops at little cafes (maybe even a pub or two) along the way. Some of those bike rides go through beautiful countryside. They will be taking regular pit stops in the middle of nowhere, to catch their breath and enjoy the view.
Having said that, I have one and only one non-female cycling buddy I'd also do lunch & sightseeing with. He's an old friend, also a good friend of DP and they would also do things separately where I wasn't interested.
So I guess for me, a cycle-training partner of the opposite sex for weekend warrior riding = not necessarily a problem. A cycle-training partner who I hadn't met and who was also involved in other hobbies, lunch etc. - nope wouldn't be comfortable.
A lot of this I think is gut feel rather than hard and fast rules tbf. DP went on a 3hr cycle with a woman from work who's a bit older than us. She was a cycling newbie and really wanted to do this route and I wasn't available. I had zero problem with this, but then again I've met her and DP was upfront about this. They also do another sport together that I am shit at.
They don't meet for a nice lunch or go out sightseeing. In the event she's having a party or night out DP is invited to, I am invited too. I think that's a big difference as she's not an unknown quantity and nor is my male cycling buddy.