Before we read the passage that will answer your question, I like how you added the words “but it is not written” after your question. The fact is, the thought that “Gabriel spent time in bed with Mary” is not written, because IT NEVER HAPPENED (as we shall see). The portion we will look at is Luke 1:26-35 (from the NKJV version) and it will be best to break it down to see, step by step, what happened.

Verses 26-28: “Now in the sixth month the angel Gabriel was sent by God to a city of Galilee named Nazareth, to a virgin betrothed to a man whose name was Joseph, of the house of David. The virgin’s name was Mary. And having come in, the angel said to her, ‘Rejoice, highly favored one, the Lord is with you; blessed are you among women.’” If you were to look up Gabriel in a concordance and read every passage where he is mentioned, you would see that he was God’s special MESSENGER to His people. Every time Gabriel was “sent by God” to someone he had a special message to deliver to them from God. As we shall see, he had a marvelous message for Mary, but we shall also see that this message was his mission. He was NOT SENT to Mary to have any kind of physical relations with her. In these verses he prepares her for the message by informing her that she was “highly favored” by God and that “the Lord is with you.” God was going to visit her in a special way and this would result in her being “blessed among all women.”

Verses 29-30: “But when she saw him, she was troubled at his saying, and considered what manner of greeting this was. Then the angel said to her, ‘Do not be afraid, Mary, for you have found favor with God.” Mary was a humble woman and thus she was troubled by the angel’s words; no doubt she thought she was not worthy to be so blessed. Yet the angel hushed her fears and repeated the fact that she had “found FAVOR with God.” The word “favor” means “grace,” so Gabriel is teaching her that God has, in His wonderful grace, chosen to use her as a “special recipient of His grace.”

Verses 31-33: “And behold, you will conceive in your womb and bring forth a Son, and shall call His name JESUS. He will be great, and will be called the Son of the Highest; and the Lord God will give Him the throne of His father David. And He will reign over the house of Jacob forever, and of His kingdom there will be no end.” Here is Gabriel’s MESSAGE TO MARY! She was the one chosen to bear the promised Messiah, “the son of David” (see 2nd Samuel 7:8-16). He would also be called “the Son of the Highest” for as we will see in verse 35, He is also “the Son of God.” This miraculous birth would result in the Incarnation of the Son of God! The prophet Isaiah foretold this in Isaiah 9:6, “For unto us a Child is born, unto us a Son is given.” The “Child” Mary would give birth to was the “Son” that God sent into the world to be the “Savior of the world” (see John 3:16 and 1st John 4:9-10, 14).

Verses 34-35: “Then Mary said to the angel, ‘How can this be, since I do not know a man?’ And the angel answered and said to her, ‘The Holy Spirit will come upon you, and the power of the Highest will overshadow you; therefore, also, that Holy One who is to be born will be called the Son of God.’” I don’t believe Mary was doubting the word of Gabriel, but she was wondering how she could bear this Child without having sexual relations with a man. Gabriel’s answer is amazing, for in essence he told her, “You’re going to experience a VIRGIN BIRTH! You will conceive this promised Child by a MIRACLE, for the Holy Spirit will ‘come upon you’ and the Highest will ‘overshadow you.’” What does this mean? We aren’t told the exacts details involved in those words, but I believe it means that the Holy Spirit would give Mary a “miraculous conception,” not by having a physical relationship with her, but by visiting her womb and causing her to conceive by His power. She remained a virgin, but she became pregnant by the Spirit’s miraculous power and nine months later she would give birth to the promised Child and she was to call “that Holy One…the Son of God.” Notice, Jesus was “that Holy One,” for even though He would be truly HUMAN (see John 1:14 and Hebrews 2:9, 14), He was not “born in sin” like the rest of the human race who were born through two human parents (see Psalm 51:5 and John 3:6a). Yet He was also called “the Son of God,” for He was and is THE SON OF GOD (as we saw above). He would be, from that point on, “God manifest in the flesh” (1st Timothy 3:16).  (467.5)  (DO)