The only precise way to translate "eos" is "prior to". It does not necessarily indicate that the opposite happened after the event the preposition controls; that should be inferred, or not, from the context. For example, if I say "I did not drink alcohol prior to the age of 18" the context indicates that after that age I did drink alcohol. That is because the mentioning of age is otherwise meaningless. But if I say "I did not drink alcohol prior to this blood test" it does not mean that I walked out of the clinic and drank. The context is about the blood test, which would be affected by alcohol consumption prior, but not after.
In Matthew the context is the supernaturality of the birth of Jesus, so he explains things in relation to that event. He is simply not concerned with Mary and Joseph as a couple.
"Eos" is used in the sense that precludes the understanding "prior but not after" many times in the scripture. For example, Luke 1:80 refers to John the Baptist being in the desert "eos" his manifestation to Israel. But we know he remained in the desert and preached to Israel till his arrest much later. In 1 Timothy 4:13 Timothy is told to continue studies "eos" Paul arrives. Clearly Paul is not telling Timothy to stop afterwards.
Keep alive.