My son wants to live with his in-laws after marriage and sign the house before marriage?

My son and his fiancee will be getting married next year. The fiancee wants to live with her parents. The fiancee s family already sold their house, and now they are looking for another house. The fiancee (she will graduate from graduate school in 2 months) cannot sign, the fiancee s parents cannot sign (legal reasons), and so my son wants to sign the house for them to live in now, and he will move in with them after the wedding.

My son will do anything for his fiancee. I initially disagreed with this whole plan. I don t want my newlywed son to live with in-laws and I don t want him signing anything before marriage. So I sent a letter to the fiancee s parents saying that I wouldn t approve of the marriage unless:

1. Her family finds a place temporarily (until she makes enough income to qualify for a mortgage) and she should sign the house
2. My son and his fiancee should live alone for the first 4-5 years, and then they can live with the fiancee s parents when they ve started a family already

Now my family is in a civil war as people choose sides. Am I being unreasonable? Are my conditions wrong to ask for? Should I let my son "live his own life?" My son is saddened because he thinks I m giving him an ultimatum (follow my rules or I won t attend your wedding), but I see it as my son leaving our family because he is blindly in love.
Added (1). I am also okay with my son signing the house after the wedding, I just don't want him to sign it before. To be fair, I know that my solution may sound a little unreasonable. While the fiancee's parents live in the house for the first 4-5 years, my son/daughter-in-law would not be able to buy a house (they already signed for my daughter-in-law's house; also there would be no reason to buy a house as they would be returning to live with the parents in 4-5 years time anyways), so they would rent
Added (2). My wife and I sat down with my son many times and tried to get our ideas through to him, so we thought it would be reasonable to express our discontent with the parents after that didn't work.
Added (3). Also, the fiancee threatened to break up with my son. However, she worded it beautifully as "I would break up with you because I want you to prove that you can fight for me, and not give into your parents." When in reality I think that means "We have to live with my parents or I'll break up with you." I understand that I'm biased, so let me know if this interpretation is unreasonable

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