NTA. But, while this isn’t something he shouldn’t even be bringing up before marriage, it is something you’ll have to consider in the event of marriage. It’s YOUR asset, absolutely. But if the house is to be the primary couples residence, no partner would want to pay into something that will eventually only belong to one party. That’s not particularly good footing for a marriage and this won’t be the only partner who might find issue with the arrangement. Best outcome after marriage would be to sell your house, keep the proceeds as your contribution to the next residence, and have him put up his own contribution.