Hey, most people have two parents. If there's been a divorce, you might have four. So the number of parents isn't really the determining factor.
The question is, how do we design the site so that users can find what they're looking for?
What I'm proposing is that there is no single answer to the question of "should a hut be a parent to a route"?
If the hut occurs in the midpoint of a route, it seems to me it's a part of the route: the only people who care about the hut are people who care about the route (but probably not vice versa). Making the route a parent to the hut is simple and logical in that case.
A hut located at a crossroads might be part of two or more routes. That works if you have long routes where the hut is just a stopping point along the route(s): clearly the hut belongs to the route(s).
But if you've got short routes starting from a hut, it doesn't make much sense to say the hut "belongs" to them. It might make at least as much sense to say the routes "belong" to the hut.
Unlike huts, trailheads are never at the midpoint of routes, always endpoints. A given trailhead is often the starting point for multiple routes, and it's not obvious whether a trailhead "belongs" to a route or whether a route "belongs" to a trailhead - it's' just a matter of perspective. Since we can't easily decide, we can just say "Related" and stop worrying about it.
In other words, go ahead and make a parent-child relationship if there's a clear, one-way sense that one object "belongs to" the other. Otherwise, stick with Related.
Example: Often a hut clearly "belongs" to just one or two mountains or a small range.