Hi there. I hope I can help.
In regards to what you're asking in the topic title, as a guy, my definition of cheating is ANY sort of inappropriate ("inappropriate" defined as a relationship or action not compatible with a monogamous relationship) attachment to an outside source, be it emotional or sexual. I think fellatio well outside of what is considered "appropriate," and is thus cheating. I also believe that unless there is expressed consent between both partners, there shouldn't be pornography in a relationship. I don't chalk porn itself up as cheating, but I think it's a gross disrespect of you for him to be looking for that behind your back. Of course, I don't think there's a need for me to pass judgement on the Facebook.

I am so sorry.
That being said, I know you love him, and I know you want advice on how to salvage things. I'm not sure what I can offer you there Poil...I think it's better to end this now than to marry him and go through this as husband and wife.

It is better to be "alone" (which I'd offer you, you're not alone...being single sucks, Lord knows I know that, but there's always people out there) now than it is to be with someone you can't trust. For what it's worth, I've tried, and it was the most hellish period of my life I can paint for you. Granted, circumstances were significantly different, but the fundamental stays the same. You don't want this for yourself.
I am sorry I could not offer you the advice you were seeking. However, for this, I feel it better to be honest than to sugar coat the matter and potentially usher you into unhappiness. Please forgive me if I seemed overly blunt or harsh. That was not my intention.
Please know I'm praying for you, and I wish you my best.
Hugs,
Harley