Chaste marriages were allowed back in those days. God did allow it, look at
Num 30. It’s all about taking vows of celebacy and being married.
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Moses then gave the Israelites these instructions, just as the LORD had ordered him.
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Moses said to the heads of the Israelite tribes, "This is what the LORD has commanded:
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1 When a man makes a vow to the LORD or binds himself under oath to a pledge of abstinence, he shall not violate his word, but must fulfill exactly the promise he has uttered.
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"If she marries while under a vow or under a rash pledge to which she bound herself,
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and her husband learns of it, yet says nothing to her that day about it, then the vow or pledge she had made remains valid.
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2 "If it is in her husband’s house that she makes a vow or binds herself under oath to a pledge,
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and her husband learns of it yet says nothing to express to her his disapproval, then any vow or any pledge she has made remains valid.
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"Any vow or any pledge that she makes under oath to mortify herself, her husband can either allow to remain valid or render null and void.
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But if her husband, day after day, says nothing at all to her about them, he thereby allows as valid any vow or any pledge she has made; he has allowed them to remain valid, because on the day he learned of them he said nothing to her about them.
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These are the statutes which the LORD prescribed through Moses concerning the relationship between a husband and his wife, as well as between a father and his daughter while she is still a maiden in her father’s house.
If chaste marriages were not allowed by God than why is the whole chapter about chaste marriages? Please do read the whole chapter. Also, keep in mind that Mary and Joseph were not under the New Convenant at the time of their betrothal. They were under the Old Convenant.