My wife and I loved it when she would bite them. It would give me an instant hard-on. She used nipple clip on them and then would knock them off with a riding crop. I do love pain.
One time I was at a camp out when 2 female friends were challenged to hold my arms down and sucked both of my nipples at the same time. Then they bit down on them. I moaned. One of the girls laughed and sat on my lap. She instantly felt my erection! She hugged me a said "You loved that didn't you!" She told the other girl who came over and grabbed ahold of me and announced to about 20 people that I got a hard-on.
Talk about embarrassed. I loved it!