I'm not sure what the problem is. Are you saying this guy is fixated on you and is just thinking you are going to be his wife and is just focusing on you? And at the same time you aren't anywhere near the same place and don't feel any real desire to marry this fellow?
If that's the case, you really need to talk to him about it. Such an unbalance in a relationship creates a lot of problems.
Now the problem might not be with you. The guy might be living a fantasy.
In any case, if you talk to him and you see him doing what ultimately will hurt him, then the kind thing to do is to break it off. You don't want him putting his life on hold for 10 years thinking you are going to change. Who knows, maybe you would, but it doesn't sound like that is likely.
As far as a dating site. It would probably be good to put your feelings about being single and marriage right into your profile. Nothing wrong with it as long as you are honest. If it's assumed you are really looking for marriage simply by being on the site, do something to make it clear how things really are.
That's what I'd really suggest, just tell him and any other guys what you told us. If the guy thinks that's okay and he would still like to spend some time with you, that's fine. Nothing says that single people have to spend every waking moment desparately trying to be married.
I would comment as for the contented single thing, that many of the best marriages are made up of people who were quite content as singles. They are comfortable with themselves and yet at some point they find another whom they enjoy marriage to. Some of those people you read about with all those struggles as singles really don't make good mates. The struggles continue after marriage, it doesn't fix those things they thought it would. Sometimes they just marry whomever will marry them. Anyway, there's a reason why most people run for cover when someone shows up who seems just desparate for marriage and unable to cope with being single.
Marv