To be fair, Joseph was trying to render Rhys completely senseless with pleasure. It was a rare pleasure to be someone's first lover. Such an honor deserved to be treated with reverence and one's best effort to ensure the less experienced partner enjoyed the experience fully. Joseph wanted this to be their first night together, not their only one. But if it was the only one they shared, at least it would give them both pleasant memories to enjoy.
The small resistance just made it better, really. Rhys was a gentle, easily guided soul. But having him challenge Joseph's sexual dominance even faintly was thrilling. When he felt the younger Kindred grab his free wrist, he applied more pressure with his teeth.
Letting off that wrist, Joseph moved slightly so he could put Rhys' other arm closer. He let the Tremere resist a bit before using his superior strength to bring that arm within range of his fangs without breaking Rhys' grip on his wrist. Since his wrist was out of range, Joseph bit down furter up the arm, on the vein the ran along the bottom.