Is my boyfriend cheating on me?

we have been dating for 5 years now and we have our fights here and there. he was using my laptop and left his yahoo page opened. as i went to look he was signed up on match.com eharmony.com and alot of sites to find a girl and have casual . i havent confronted him since i know i shouldnt have been nosey but now that i know, is he cheating on me??