Boyfriend cheating online

So we've been together for many years, engaged and living together.. He is very loving, attending and supportive man, he says he wants us to get married, have kids and I want it too..

But throughout all these years I constantly caught him messaging random women on social media, flirting with them, saying he is single and asking them to send him naughty photos (lots of them did)..

Every time when I ask about these messages he says he never spoke to them women and doesn't know them (which is lies, because I saw these messages and photos on his phone *and I'm not proud of it*)

After another fight he apologies, promise he won't do it again, says he loves me and never was interested in them women and never intend to meet them (which I think is true because he used to message random women and from another country or far away city)

But he always comes back to it and keeps lying about it every time... When I try to talk to him and explain why he does it, he says he doesn't know why, he just does it...

So any ideas on what is happening? Is it some kind of sexual addiction or he just doesn't care about our relationship and want to break up? Because I am about to pack my suitcase and leave

Trying to change other people, even with consequences hanging over them is just about impossible.
If he was honest with you (and himself), I might think that there was a chance for change. If he can't be honest, then the chances of resolving this is poor.

If he doesn't see the problem for himself with his own behavior, there won't likely be change.There is no real incentive for him to resolve this. You share this problem with many others.
He needs to ask for help as he is stuck in a loop. He probably won't be able to stop on his own.

Unless there is an Epiphany where he sees (or feels) how deeply this is hurting you, it won't change. So far your hurt hasn't stopped him.

Best of luck to you.

I don't believe on online dating.

the harsh truth of life is this - you should not waste time on this person because he is constantly looking for an alternative to you. whatever you do, it will always take place