Since the question is about the trousers themselves getting the semen, it probably means lack of underwear. Assuming this is true and you sleep without underwear, my tip is to wear underwear beneath the trousers while sleeping.
This way the semen will be absorbed inside the underwear and won't reach the trousers. In case of large volume that goes through, just put another pair of underwear on top.
While you will still have to clean up every day, cleaning up underwear should be much easier than trousers.
After giving it some more thought, I decided to go the extra mile and add tips regarding the underwear themselves. There are basically two main options:
- Underwear that are meant to absorb liquids while being washable (also known as Incontinence Briefs), it will make sure no semen will leak at all no matter how much you'll have. Downside is that you'll still have to wash them daily. From quick look around, this brand appears to be good, it's not cheap but you buy three pairs at once, one time, and can keep using them for as long as you want.
- Disposable briefs: you wear each pair once and just throw it away instead of washing it. It's originally meant for people in hospital/sauna/etc where you can't wash the underwear, but can't see reason why not using them on regular basis if you can afford it. Quick looking around gave this, for example: five pairs in the pack, for reasonable price. If washing the clothes is a big concern, this can be the ideal solution. (Keep in mind that those aren't meant to absorb liquids though, so like I said before, in case of large volume of semen that gets through, wear two pairs at once.)