Therapists are trained to be empathetic, understanding, and non-judgmental. They are accustomed to dealing with various issues and situations that their clients may bring to the therapy session. If a client were to accidentally stain the therapist's couch during their period, a professional therapist would likely respond with compassion and understanding.
It's important to remember that menstruation is a natural bodily process, and accidents can happen. Therapists are generally prepared for these kinds of situations and are not likely to be angry or find their client gross. Instead, they would focus on supporting the client emotionally and ensuring that they feel comfortable and safe during the session.
If you are concerned about such an issue, it might be helpful to talk to your therapist about your worries beforehand. This way, they can reassure you and take any necessary precautions to make you feel more at ease during your appointments.
Remember that therapists are there to support you and help you work through your challenges, and they understand that life can be messy sometimes, both literally and figuratively. So, try not to worry too much about it, and if the situation were to happen, a professional therapist would handle it with care and understanding.