What is incumbent upon me if I have sworn not to marry, and must expiation for the oath be performed before marriage for it to be valid, or can it be performed afterward without affecting the validity of the marriage?

It is clear that the whole matter is nothing but whispers (waswas), and that the questioner did not utter an oath, but rather his soul spoke to him. Therefore, he must ignore these whispers. And even if we assume that he did utter an oath to abandon marriage, he is permitted to marry and expiate his oath, and there is no harm in expiating it before or after the marriage. This does not affect the validity of the marriage; in fact, even if he does not expiate at all, the marriage remains valid.