I don't ask them their relationship situation. I figure if they want me to know they will bring it up. As outcall often it is a giveaway if they are single, or have a partner. It is their business though.Now I'm curious about the reverse. Do you ask clients about their relationship status? Do many clients bring it up themselves, unprompted? Is it information you want to know or not? Does it change anything?
I'm guessing it's more common and acceptable to discuss the client's relationship status since the SP/client relationship can often be similar to a therapist/patient relationship.
I have had sessions that were more like therapy, either because they were dealing with mental health issues I can relate to, or just want someone to hear them out, or feel close to. I've also done some relationship advice when issues in their relationship come up.