I don’t really understand all the “buying a house with someone you’re not married to” comments. A relationship is a relationship. You can create the contract in a way that you go out with what you came in with, should you split in the future.

As far as loaning her the down payment, it’s a little odd to me that you would be “loaning” a partner you’ve been with for five years anything. I make more money than my partner, and have been able to provide more financially to our relationship over the years. But there were times when it went the other way. So I’d just ask you, if things were flipped, or if they eventually flip, would you expect to be treated the same way by your partner?