If two (or more) people want solemnly to declare that they will never leave each other no matter how much they may later want to, that should be their right

This idea is not quite clear to me. If I marry someone and, in addition, declare that I will never ever leave my spouse that I should be obliged to keep such promise? I thought that certain types of contract conditions should be void under modern legislation. If I want to sell myself into slavery and noone forces me to do so but nevertheless I wish to sign for it - this condition should also be void by any means. Even if I signed it, even if I got money in return for that - still noone should be able to enforce it. Catholic marriage, slavery, peonage etc. are inhumane things and shouldn't be considered as valid conditions under any curcumstances. If two same sex people sign for an eternal marriage - it doesn't change a thing.

Is there anyone to disagree on this issue?