I think I understand what is going on with Sean. He has been looking very stressed for a few days now. I reckon he doesn't feel the same way Mark does (and to be fair, he has had enough time to know that by now). Mark has piled on the pressure from the beginning about not wanting to be hurt or abandoned, and if poor Sean knows that he doesn't fancy Mark, he must feel trapped and knows he is going to upset someone, be made to look like the bad guy, just because he feels the way he feels. He is allowed to have his own feelings, but Mark seems the type to turn it into a drama sympathy-fest which will isolate Sean. That's not fair.
Sean needs to be honest, tell the truth, and let the experts and psychologists sort Mark out. I know how Sean feels as I have been in a relationship where I felt trapped, too much to soon and under huge pressure. You feel like you just want to run.