They may start to wonder how many other guys you have had sex with so quickly, even if you have only been with one or two.

The number of guys you have had sex with does matter to a guy looking for something long term, it doesn't matter so much to a guy who is just after sex in the first place though.  And he will think you've been with loads if you have had sex too soon with him.

It may also make them think you may cheat.

That's very perceptive, a man who wants a long term relationship, doesn't want to be with a woman that is more than likely to cheat on him, even if she is super attractive looks wise.  And having sex too soon will more than likely make him think that she'll turn into a Desperate Housewive type.

I personally think though that starting with a nice friendship, building up trust and respect and letting things progress slowly will help cement the relationship and give it longivity.

That is what I'm looking for and what I would expect other guys who want a long term relationship to be looking for.

If the anticipation is over too quickly you haven't give the other feelings time to develop and a successful relationship will thrive on things moving in a set order.

You do sort of scratch your head and wonder what else you can do with her if you've already had sex.  You've skipped the other steps and worked your way to the final point.