“Max leaps into someone’s eyes as they are jerking off whilst Max thrusts into him.” OK, maybe that’s not verbatim a line from Finnish writer/director Mikko Mäkelä’s “Sebastian” screenplay. That’s Mäkelä demonstrating how specific his script was about the gay sex he wanted to make happen onscreen. This provocative drama, a Sundance 2024 premiere, stars newcomer Ruaridh Mollica as Max, a 25-year-old writer who submerges himself in London’s underg…