proposed marriage toThe に marks the person being proposed to. 結婚を申し込んだ is the past form of 結婚を申し込む, meaning “to propose marriage”; it is somewhat formal and can also be expressed as プロポーズした.
JA: 彼は長年付き合っていた彼女に結婚を申し込んだ。
EN: He proposed marriage to his longtime girlfriend.