Contract magic is a magic that works on the basis that both parties fully agree to the terms of the spell cast, meaning that any form of magical coercion invalidates the magic itself. (That said, physical coercion is not subject to this restriction if the subject isn't scared silly to the point that he can't read the contract to the letter. Alcohol and drugs can also bypass the restriction if the victim understands the contract but his judgment is impaired.)

That means that if someone is adamant about not surrendering, or if you give terms that the other party will find worse than death and torture, you can forget about forming a contract with the said individual.

Also, the contract itself is subject to a strength factor, depending on how much mana you infuse into the contract, and also the possible punishments in the contract.

If it is a 3rd circle barebone contract, the contract can't outright kill you, but it might take away other things, such as 10 years of lifespan, knowledge in the brain(Which can also be Skills), 100 gold coins in your possession suddenly teleporting away, some of your hard stats(Strength, vitality, intelligence, etc.). However, it still cant be a Faustian contract, as those contracts are 5th-circle and upwards, and it cant kill you in some horrible way since that is a 4th-circle clause.

Moreover, an individual with enough strength AND willpower might be able to break or mitigate some of the contract's power. However, the amount of mitigation is very dependent on the punishment's nature. If a demigod was subject to a 7th-circle contract that stipulated that he needs to give up 5 points of strength when he breaks the contract, he might be able to partially resist it and only lose 3 points of strength. A death clause might result in a long coma. If you place a divine artifact as collateral, then instead of losing the item, it might get damaged to an extent, or some broken parts of it might get claimed, although if you bet some gold coins, you will likely still lose them all. Eternal servitude might end up as a temporary servitude or heavy damage to the psyche and soul.

Soul bargains, however, are hard to resist, as the soul itself is relatively fragile and weak. If you lay a valid claim on a soul, even gods will only be helpless to watch as you claim the soul. (There is a divine court on this matter if someone wishes to appeal on behalf of a soul(If it is still intact), but the judge will almost always rule in your favor, not your victim's god. Even if the god wins the appeal, he needs to give an equal or greater compensation.)

There are also ways to try and remove(Not break) a contract. If the contract is too weak, say, 5th-circle, an archmage might be able to find a way to remove it through various means, both on himself and on other people(That is, if he is willing enough to do that, something that I find hard to believe). However, if it is an 8th or 9th circle, he probably won't be able to do anything against it unless he has something extraordinary to remove it(Like a gods intervention, which again, I find hard to believe since there are divine rules that restrict such actions), and can only suffer the full brunt of the contract and all of its ramifications once it is broken or completed.

If a contract is already completed or broken, even a god will be subject to its effects, however weak it is, but he can still attempt to resist it partially.

What I am trying to say is, that contract magic is STRONG. VERY, VERY, VERY, STRONG. A contract is a contract, and every debt has its collector. Ordinary folks are usually not subject to it since they are not worthy enough, but strong individuals can and are subject to it.

Note: That marriage thing you mentioned actually doesn't happen, especially in long-living races, because they view romance as a momentary passion, not something everlasting. 100 years is considered a long and lifelong marriage, but elves can live up to 750+ years, some even thousands of years. Your average and devout elf might remarry 5-10 times in his lifetime. Moreover, the contract itself might lead to a dent in the relationship between the two, becoming a counterproductive shackle that makes both parties tense and even hate each other. Having a marriage contract is an idealistic fantasy that just doesn't exist, as life doesn't work like that, and people know it.

Note 2: Contract magic has the power to claim and restrict things, not change someone's psyche. If you write a contract that says that both parties will be a loving couple, they might act like it, but it will be all an act. An eternal servant contract is a contract in and of itself, and the servant might not be devoted to his/its master, but might resent them to the core, but be unable to do anything about it.