The way I see it, your trousers getting dirty might be the smallest problem. When an old man starts having sexual desires again, he is immediately sent to have a prostate exam - and for good reasons.
So the first thing that I would do - I would undergo some medical tests to see if there is a problem of the body. Emissions EVERY night is not so common, afaik.
Additionally, especially considering the sexual characteristic of the problem: I would find a counselor (trainer, psychologist, call them any way you want), to find the cause of all those dreams and of all those emissions. You might be in for some interesting surprises there :)
Other than that, other users already answered the basic issue: wear underwear, wash often. Maybe even wear 2-3 layers of underwear during the night.
Ultimately, wear a plastic bag over the underwear, to restrict the spread of the emissions. You can make that from a 30L garbage bag, in which you make 2 additional holes, for your legs.If you are careful to wash and dry it, you can use it several times and be friendly with the environment.
The extreme idea is to wear a condom during the night, but that will most likely lead to other complications.