since they don't consider themselves "friends" he might think that they are indeed not close to each other
and given how fucked up he is, he probably wants Sena as a friend (even though they obviously are all friends by now) and everytime they come up with their desire to do the deed he tries to outmaneuver them, because if he doesn't then he'd simply bypass the friendzone... not to mention that it would leave a stain on everyone else - and I'm not talking about jizz