Casual long term dating

I am 23, and I've been casually seeing someone for about 7 years now. I don't like to commit to a single person, generally, and he feels (or felt?) similarly.

Despite the multiple relationships we've had during this time, we've developed an intense fondness and understanding. But sometimes, it grows hot or cold. Sometimes we go months without speaking to each other, which I find fantastic. The troubling thing is he recently asked me to marry him. I turned it into a joke, mostly because he scared the sh*t out of me, but also because I think he is desperately trying to possess somebody because he's 28 and growing insecure about his future. We've secretly loved each other for years, but I'm really wondering how to handle this situation. Any suggestions?

It's time to call it quits!

Hello,

Despite not knowing you, reading your message I immediately picked up on uncertainty - you are uncertain, not about how you feel about his proposal, because the simple truth is you have no desire to marry this man, but rather uncertainty of what will happen when you say no.
If you say no, that opens up the possibility of you no longer using each other as a 'crutch' for ever, however, you were both playing with fire when you thought that you could have your cake and eat it, because the simple truth is; no matter how much we can promise others and even ourselves that a "deal" is simply that, it never is as simple as that. In the vast majority of cases, I'd say 99%, one or both parties are definitely going to get hurt.
You have a bond, and what you considered a mutual understanding; you were "play mates" not "soul mates" and that suited you both, however, your hand has changed and you now must accept the situation and play with the cards that you have been dealt.
I have no doubt that you do both love each other, but there are many types of "love" and he isn't your "can't live without him, can't breathe without him, the type of love that is so consuming and you truly believe that you will grow old together" you don't feel that for him, you truly wish you could, but the simple truth is that you don't. I can even guarantee that you've thought "but maybe...over time.." It doesn't work that way.
He has now reached the "OMG I'm almost 30!" stage and society has imprinted in us all that by 30 you must be married or somehow that defines you as some sort of social dysfunctional outcast or a tragic case. This isn't the case, I do believe in marriage but only when you know for absolute certain "this is the person I am supposed to marry" and you don't feel that way about him.
Have you also considered, because you care for him that in a way you almost don't want to say no because you fear of how this may "crush" him? The truth is it will;temporarily.
You aren't supposed to be together in that way, because if you were you would never have posted your message, seeking advice from people you have never met. You know, deep down, he isn't the guy for you, and you aren't the woman for him either.
I honestly believe you should tell him the truth; you love him but not like that.
I think you two are supposed to be life long friends that occasionally have 'ex-sex' when you have relationship breakdowns to feed each others egos, but nothing more.
You're soul mates - but not on "that" level.
Also, at 23 - you are too young to consider marriage, I am the same age and won't even consider it until I'm at least 35 because I know that I won't be who I am right now in twelve years time so realistically to get married at such a premature age is a recipe for disaster and broken hearts all around.
Mull over your thoughts and feelings and the answer will come to you.
And remember, there is no shame in saying; I love you.....but I love ME more.