Responses
well, it's not incest, but methinks Byul believes it would be ungrateful to rob them of the chance of having grandchildren and throw everything they did for him back in their face by doing ecchi things with their real son and thus both their children will be gay and not marry and do all that white-picket-fence family expectation stuff.
Yes they are not related by blood but it’s seems that Byul sees him like a little brother so Maybe for him it’s feel like that
well they aren't related by blood, so i don't think it's an incest, more over i don't think anything will happen, byul (is this his name?) seems not to like him in that way ╮(￣▽￣)╭