Smh fetish or nah?

So....may be tmi but anyways my fiancé has been asking me for a while now( LIKE THE PAST 2 YRS) if he can do it in the back ( butt) but he only ask me during intercourse and I always say NO so we don't talk about it otherwise! so last night was my opk so we bd and he just inserted it in there and I felt totally violated so I told him no and that was the end of that but idk why he wants to do it there its gross (to me not judging if you do it) but is that even safe? Does anybody else SO do this or want this type of pleasure? And it HURTS! 😱😨😥😒🙏