The forced acceptance of Torah at Sinai was to make it irrevocable

from Daas Torah - translation copyrighted

Maharal (Shemos 19:17): … I found in a medrash that when G d  gave the Torah He forced the Jews to accept it. Since the law of a raped woman is that she can not be forcibly divorced, consequently G d can never reject the Jews. It is important to note that the medrash is not saying that the covenant at Sinai was a form of rape. We obviously can not learn from the sinful conduct of human beings the nature of the covenant with G d. However, the medrash is teaching us that the punishment the Torah gives to a rapist is that he can never divorce the woman — and not some other punishment — because the punishment is appropriate to the deed. That is because every deed which is forced is irreversible while a voluntary deed is reversible. Thus the forced relationship with a woman is also irreversible…That is the reason that G d’s relationship with us was done in a forced manner so that it would not be reversible. Therefore even though the Jews had initially accepted the Torah willingly by saying “We will do and we will understand” — the essential relationship was one that had to be and thus it had to be forced.