There is not much difference between adopted and biological. If his parents insist that the adoption is not as good as their own, or discriminate that you are adopted, it is the problem of the husband's family. Then you have to think about whether such a husband's family is worth going to.
If you are adopted, you won't have an accurate date of birth, and they can't detect your horoscope. You can go with your boyfriend to discourage the elderly.
Middle-aged and elderly people are generally superstitious. You can make it clear to them that these are unscientific. Whether love is suitable or not depends on whether two people's personalities are suitable or not, and whether they will consider each other humbly. But you can't refuse directly, and you can solve any problems you encounter. They are also thinking about the big wedding, and the old man is stubborn and understandable.
Explain to them that even if you were adopted, you were raised by your parents since childhood, no different from your own. They gave you a good family education, and your personality is fine. You can be a good daughter-in-law they can trust.
If they are still uneasy, then you can suggest that they spend more time with you before deciding whether you want to be a wife. It is unfair to you two to judge whether you and your boyfriend are suitable simply by looking at your face or horoscope. After all, superstitious things are unreliable. Only by really getting along with you for a while can you feel at ease, and you can't simply listen to superstitions.