If you research Marek's you will read a lot of scary stuff. In my opinion that is because the majority of cases that are diagnosed are the severest strains where it has killed multiple birds and hence has triggered people to get a necropsy, but there are many flocks with milder strains that are never diagnosed. They get a lame bird here and there or a bird with wry neck. Some recover with vitamin therapy, some don't and some deteriorate and die but never get necropsied. Everyone believes it can't be Marek's because the severe strains they have read about kills lots of birds, but it is an extremely common and widespread disease which is very easily contracted and due to the dormant periods and variety of symptoms it is very hard to correlate one symptomatic bird to another. It is a Herpes virus and similar to the Cold Sore Herpes virus in humans a large percentage of the population have it but only a few show symptoms at any one time and they are only infectious whilst they are symptomatic. There is a minimum 3 week incubation period between infection and becoming symptomatic but it can be considerably longer, so birds don't all become symptomatic at the same time, just here and there.
Your young male is reaching an age when his hormones will be raging and he will take whatever opportunities to mate that he can, whenever the main rooster's back is turned and he will try to mate with any pullet or hen he can get hold of. This girl will almost certainly become a target for him because of her reduced mobility, regardless of whether she has started laying or not. Sexual frustration will drive him to it sooner or later in my experience and opinion. In fact he may already have started but you just haven't witnessed it yet.