Generally, people who enter the U.S. under the visa waiver program cannot change status in the United States but if they are to change their status based on a marriage to US Citizen, they CAN change their status without any problem. The only thing that you need to be careful about is not to get married upon arrival. An immediate marriage upon arrival with visa waiver problem can bring problems. Date him more and get to know him better, then get married. Good luck.
You are also missing an important piece regarding intent and what the declared and presumptive intent is when using the VWP to enter the US.
http://www.uscis.gov/files/article/A2.pdf
What if my fiancé(e) uses a different kind of visa, or enters as a visitor without visa, to come here so we can get married?
There could be serious consequences. Attempting to get a visa or enter the U.S. by saying one thing when you intend another may be considered immigration fraud, for which there are severe penalties. Those penalties include restricting a person’s ability to get immigration benefits, including permanent residence, as well as a possible fine of up to $10,000 and imprisonment of up to five years.
It is not appropriate for your fiancé(e) to enter the U.S. as a visitor with the intent to marry you and remain to try to become a permanent resident. It is appropriate, however, to enter as a visitor to have the wedding in the U.S. and then return to a foreign residence for further processing for U.S. immigration as a spouse.
You should come prepared with proof of your clear intentions in this regard.
If I had answered this post first, besides asking questions about what they meant to do after the wedding, I would have reminded kindred that it is illegal to enter the US as a visitor when you actually have the intent to immigrate (marry + adjust status).
PS: Momeni isn't the only case floating around on this, tho it is frequently discussed. There are starting to be user posts about removals.
And I don't see the logic in the 'don't marry immediately'.. can you flesh out what you mean by "The only thing that you need to be careful about is not to get married upon arrival. An immediate marriage upon arrival with visa waiver problem can bring problems. Date him more and get to know him better, then get married."