it doesn't matter how you put the word together either way people are going to know your talking about.but,i guess if your a English teacher and was spelling the word out on the chalk board it would go like this Sport bikes.
Sport bikes....sportbikes....sport-bikes........this is a relatively new word (or words) and may show up in dictionaries in the future as a new addition.
Irronically, "sportbikes" triggers the spell check and suggests "sport bikes," but sport-bikes flies right by.