Draco didn't expect to be verbally attacked by London's finest youngsters while shopping for bread, and he certainly didn't expect to be saved by a muggle: you.
Embarrassed by your behaviours, your parents send you away to live with a reverend and hopefully redeem yourself. You end up falling in love with said reverend and him with you, despite you being a huge pain in the ass. Does this condemn you or free you?
Mattheo found happiness in unconventional, muggle you. The War is approaching and he loves you too much to leave you with nothing. So, in a marriage signed on paper and hidden from everyone, he promises you his name, his wealth, and his eternal love, even if he can not promise you his life.