0.o at the thread necromancy.
There's something both erotic and spiritual about it. The fact that she (or he, I suppose, but I don't get the same level of that with other guys) would not only allow me to put a part of myself into her, but also that she'd want me to leave something of myself inside her, afterward? That's...always been rather mind-blowing to me.
And, somehow related to all of that, the act somehow doesn't feel complete, if I cum outside either him or her. It just feels, internally, as if something broke or went wrong with it. Cumming inside while wearing a condom is...well, it's still not right, according to my animal brain, but at least it's finishing inside, which is a good thing.
That said, if they say "Don't cum in me," that's it. Their body, their choice. No argument. Well, I might ask whether they're sure, but beyond that, no argument. Period.