Yes, it's cheating. It's an emotional or internet affair and it's very hurtful to your partner. I've been there and was FLOORED when I found out. In fact, when I found out about it,I started digging through everything (before that I trusted him 100%). I even contacted some of the women as him and made sure they never met. He made excuses and got defensive and I walked away.
A month later he called me crying, saying he had a sexual addiction, was going to counseling, how sorry he was blah blah blah. I told him I was happy for him and I hoped he got the help he needed but I no longer trusted him. A couple weeks into it, he asked me to go with him and I declined... a few weeks later I finally agreed to go. After a few weeks of counseling together we slowly began to try to work things out. (oh and a second counselor bc the first one was totally useless.... she just listened, gave no advice or opinion). I told him I expected full disclosure, immediate termination of all of those sites, and open access to everything. He agreed. For 6-8 weeks random women still text him and he learned to include me (my name/ status) in every response. For example if someone text "Hey, what are you doing?" He responded, "Cooking dinner for my girlfriend" or something like that (even if he wasn't- usually just mentioning me and they stopped). I checked his phone regularly (he used that way more than computer), recovered deleted text, etc. He knows social media and dating sites got him in trouble and I think he knows himself well enough to know he gets in trouble on them so he stopped.... he doesn't even go on FB anymore. It took time and a lot of work but he realizes how much he hurt me (he didn't see it as cheating at first. I pressed on and the counselor agreed- if he hid it from me, lied about it, or didn't want me to know it was a form of cheating).
I told him if he wanted to watch porn, that was fine, but if he ever talked to another woman again in a sextual manner (online, text, phone, in person, etc.), it was over.
It doesn't sound like your bf truly regrets what he did yet and is not truly remorseful (sounds like he's still in the defensive stage where he tries to justify his actions). Why did he do it? Only he can answer that. I think mine did it bc he enjoyed the attention, liked the excitement, and it boosted his self esteem (that's all wonderful but wasn't OK to do since it hurt me so badly). I think some men truly get a kick out of the dating sites and it's almost an addiction- a strange form of entertainment and online they can be anyone they want to be (again, I think it's an ego thing).
Regardless, it's wrong. He lied, hid it from you, and violated your trust- which is all a form of cheating. It seems like he is being defensive instead of taking full blame (another common trait these men share). Sadly, it may take losing you for him to realize how badly he screwed up and for him to take responsibility. My opinion is put your foot down now bc I personally think if you don't it will happen again and next time it could lead to something physical.
Best of luck to you!