Astarion will live forever. He just wants his love with him. And as soon as he turns Tav, he says he gave you a drop of his blood. So you're not a spawn for long, you're a true vampire.
If he doesn't have high enough approval of Tav, then he doesn't trust Tav enough to turn them, and if he doesn't love Tav then he probably doesn't want them around for eternity. I think it makes sense.
