In many Harvest Moon games, you need more than a larger house and a 'Big Bed' if you wish to marry. You need a kitchen as well. I have not listed a kitchen as a requirement for marriage previously in Island of Happiness, but it may be one of the basic prerequisites.
I believe now that you do have to have all main characters on the Island in order to be able to marry. Every player who has been unable to marry has not had a major character. This information will be added to my guide.
It actually makes sense. I wrote a warning about not marrying in haste because some of the characters do not even arrive until later in the game... apparently the game is set up to force you to at least MEET every one, giving them some chance to impress you!
The confirmed requirements for marriage therefore are as follows:
1. Experience all Heart Events: Introduction Event, Purple Heart Event, Blue Heart Event and Yellow Heart Event; (Events vary according to your response during the Event; if you make the WRONG response during the Yellow Heart Event, you may need to experience a Recovery Event as well);
2. Have the Eligible Girl/Bachelor at 60,000 AP, deepest red heart level;
3. Have the Large House and buy the Husband/Wife Bed from Chen;
4. Have the Harvest Goddess at minimum 2 hearts and make certain Alisa and Nathan have not left the Island;
5. Have all main characters on the Island;
6. Buy the Blue Feather from Chen and experience the Popping the Question Event;
Actually, having a Kitchen IS a requirement, in a sort of oblique fashion, because you cannot have Pierre on the Island if you haven't obtained five Recipes and you never will obtain any without a Kitchen! Pierre is one of the main characters and you need to entice him onto the Island as well as all other main characters in order to marry...
To have all main characters on the island, you need to pursue ALL activities. Denny and Lanna arrive only if you have done quite a bit of fishing, Sabrina's arrival requires mining, Pierre's arrival requires Recipes and the unlocking of all areas, Shea's meeting requires the bridge to the Jungle and Mark/Chelsea only arrive when you have over 50% completion... so marriage can't be your only focus.