My son-in-law is 25 years old. What is the Islamic ruling on admitting my son-in-law into my marital bedroom with a separate bed?
According to the verse 24:58, even those who are not yet pubescent must take permission to enter your room. Admitting an adult in to your room permanently cannot be allowed according to this verse:
“Oh you who believe! Let those under your guardianship, and the ones among you who have not come of age ask your permission (before coming into your room) on three occasions: before fajr prayer, in the afternoon when you take off your clothes (for sleeping) and after the isha’ prayer. These are your three times of undress. Outside those times it is not a sin for you or for them to move about attending to each other. Thus, God makes clear the signs for you for that God is all-knowing and judges rightly.” (An-Noor/The Light; 24:58)
However, if you divide the room into smaller sections either by curtains or by other types of separators, provided that you and your wive are definitely not visible to your son-in-law, then you can admit him into the same room.