The 'baggage' for me is all the cultural and social expectations around the idea of a wedding.
So if DP and I decided we wanted to sign a contract to tie up some legal loose ends around inheritance, tax etc, and we just went ahead and did it as we would any other legal or financial matter (like when we got our mortgage) - all hell would break loose when our families found out.
For some reason, because signing this particular bit of paper is called 'marriage', there will be all kinds of expectations. Our parents, siblings, cousins, friends, colleagues etc will all expect to be invited to watch us sign it (and say our brief statutory words). They'll expect me to wear a dress (which I hardly ever do), not just any dress but an expensive, impractical ballgown, and uncomfortable shoes, and get girly stuff done to my hair and face and nails and who knows what else, they'll expect expensive jewellery to be involved (I don't wear rings, ever, don't like them), for photographs to be taken of us, very bizarrely they'll expect me to change my surname that I've had for 30 years to a different one. They'll probably be expecting music and poetry of some sort, and maybe cut flowers. They'll expect us to host a big party and provide food and alcoholic drinks and play music, they might (horror) even expect us to dance, for some reason they may feel an obligation to buy us household appliances and crockery that we don't need or want.
Of course we don't absolutely have to do any of these things, but if we don't, I'll have to explain repeatedly why not, there'll be massive pressure to change our minds, so much that we might end up giving in one issue at a time just to stop the harassment, people will be offended if they're not invited etc.
These expectations are all totally alien and bizarre to me - I imagine it taking 5 minutes after work one day to say our statutory words in front of a couple of witnesses, don't care who they are, and sign our bit of paper, all done. But because it's called 'marriage', that will not be considered OK.