For me, I have a 40DD bra and it's really easy to suck on them while masturbating. It's completely normal! If anything it'a just brings even more pleasure
Let me explain.... with the hand I'm not using to rub my sex I use to push the book higher up therefore when I bend my neck down I am able to suck my tit
Perfectly normal but you do need to have big tits to do it: I find it puts my neck out of joint after a while, so I only do it a little bit at crucial moments. Of course, I'd much rather have someone else do it for me