I would guess you feel this way (romantically) because they understand and know what it's like to self harm which is something you both would share and have in common
rather than getting with someone who wouldn't understand and you would consider hiding it from or worrying about them leaving once you tell them about your self harm or when they find out about it so I understand that as a person recovering from self harm
I cannot explain or understand the sexual aspect though although I have encountered many people on the internet (usually hiding out in self harm vent spaces) who feel a sexual attraction towards it or making people or watching people harm themselves for their own sexual gratification
usually those people aren't self harmers themselves from what I've observed which is a bit interesting and strange or either it's because they are into ero-guro and knife play
or they were exposed to many forms of fictional and real gore at a young age and having that content heavily impact them
sometimes they harass other people venting about their self harm or people who post pictures of their wounds but I don't think it is as a big a problem now as it was a couple years ago not sure will definitely have to do some research