your parents must learn to accept you for who you are, my recommendation is to cut contact with them if possible, or if you can't, just refuse their ultimatum.
your orientation is part of who you are, God created you this way, there is no changing that! they can say what have you, but it won't make a difference in how you feel! if you accept their request, then you will be lying to yourself, refuse and hold steadfast.
they will learn eventually.