Adventists are true Protestants in the sense that they protest whatever is Catholic. Catholics believe that the mother of Jesus, Mary, had no other children apart from Jesus. Some Adventists, therefore, make it an article of faith, a doctrine, that Mary did indeed have other children.
Nowhere does the Bible state that she had other children. Nowhere does the Bible state that she had sex with Joseph – ever.
The Adventist Review [IA link] has an article by Angel Manuel Rodriguez from the Biblical Research Institute of the General Conference entitled Jesus’ Family Ties [Internet Archive here], which asks the question Were the brothers of Jesus mentioned in the Bible sons of Joseph and Mary? [IA link]
The answer is a well-balanced look at the facts, and says that this is, for Adventists, a historical issue, not a theological one.
The problem is that Adventists have turned it into a theological one in order to dispute Catholic teaching. Catholicism influences Adventism more than one would think.
Matt 1:25 reads as follows in the King James Version:
And he knew her not till she brought forth her first born son: and he called his name Jesus.
So the article [IA link] is not accurate there – it is not reporting what the Bible tells us, but rather it is interpreting the Bible according to the author’s own tradition. The Bible does not tell us that Joseph had sex with Mary after Jesus’ birth. It says that he did not have sex with her before his birth. The specific use of the term “until” here does not imply what most of us understand by the term “until” when we commonly use it in our everyday English speech. I say “I will wait here until I get fetched” and imply that after I have been fetched, I will no longer be waiting there.
But we can also use the word “until” this way: “They won’t repossess my car until I stop making payments.” Nothing tells us that they (the bank, etc.) will repossess the car after I stop making payments. I could stop making payments because I have paid everything, and I own the car fully.
The Bible often uses the word “until” in the same way.
(Gen 24:19 KJV) And when she had done giving him drink, she said, I will draw water for thy camels also, until they have done drinking.
After they are done drinking, obviously she would stop drawing water for them.
(Act 23:1 KJV) And Paul, earnestly beholding the council, said, Men and brethren, I have lived in all good conscience before God until this day.
I doubt Paul intended doing otherwise after that day, so it’s an open-ended until. Nothing changed when the time specified by “until” ended.
(Jdg 4:24 KJV) And the hand of the children of Israel prospered, and prevailed against Jabin the king of Canaan, until they had destroyed Jabin king of Canaan.
Did they stop prevailing against him after they killed him? To say no would be a strange answer.
The same goes for Mary. “Until” doesn’t necessarily mean that things changed after the specified time.
Another question worth thinking about: When Gabriel told Mary she would have a child, she knew she would shortly get married, and so the obvious conclusion would be that she would have a child after she got married. So why her response? She said “How shall this be done, because I know not man?” (Luke 1:34, KJV.) It makes sense only if her intent was to not know man, now or ever. If she had not yet known man, but would once she was married – and clearly marriage was the intent at the time – there would have been no such confusion.