I GOT CALLED UGLY TODAY..help?

A few boys in my school, like me and my friend, they always stare and whistle at us! Even though they hang around with theyre own group of friends who r also girls, they seem to like us.One even was close to asking me out on a date, and his friend tried to get it on with my friend too. Lol however something really weird happend today, it was a special party at my school me and my friend put on a maxi dress and were looking extra cute so we decided to walked pass the group of boys not so they could look at us but becos we had to get to the other side obviously and when we walked passed they whistled...But we ignore them every time becos tbh were not really interested in them so one of them ended up saying 'err ugly biches' ....WHY did he say this? Like seriously, what was the need to insult...Is it because he got rejected by both of us? And the guy who said it, isnt the one who likes me and my friend, his friends like me and my girl and they were sat right next to him when he said it and they did nothing. Can some body explain why he would call us both ugly? I was kind of hurt by this...