if bro cares so much why doesnt he care that he's hurting him over and over with this "sex...

Maybe 'cause he himself doesn't experience pain or emotions like regular humans do? He's practically a sentient zombie with free will and minus the decomposing.

Not to mention his mother never raised him and the one person who, if you can even call it raising, tried died in front of his eyes when he was a toddler.

You can tell in general that he doesn't really have a concept of pain given how quick he is to chop limbs off or hurt other people. He himself probably doesn't feel much considering he can change the form of his own body.