A pre-nup is usually an agreement for how financial assets will allocated during the marriage and be divided if the marriage is dissolved. Generally it's used when one person is bringing a lot more assets into the marriage than the other, as would be the case with Lex and Lois. (It could also be used to protect the financial interests of children from a prior relationship.) If the marriage never takes place, the pre-nup never applies. And I can't imagine a court upholding a contract that says one person is forced to marry another upon threat of penalty. The mere existence of such a contract would imply coercion.
In the example you cited, about who would pay any bills, I could see a contract like that being enforceable. But I also think that, on a reasonable person scale, if one party had vastly more assets than the other, the second party would 1) be crazy for signing it, since it would be very coercive (and IMO, a big red flag for even being introduced) and 2) potential grounds for going to court to say it was coercive should the terms ever come to pass. I don't know whether the second party would win or not, but I'm sure he/she would have no trouble finding a lawyer to take the case.
As for how pre-nups work in general, typically each person has their own attorney to represent their interests. Even if one person doesn't want to use an attorney, sometimes the other attorney will insist on it, so that the first person can't go to court later and claim it was unfairly created, etc. (That was the situation with a friend of mine, who was willing to sign her pre-nup without representation, but her husband's attorney insisted that she get one.) Once the terms are agreed upon, each party signs the contract and it can then be used to settle whatever applicable disagreement comes up.
HTH!
