Can you divorce your spouse?

Is there really no way out?

Correct. The wiki's Marriage page states plainly that as of version 1.0 there is no divorce mechanic in the game.

Its Spouse items page goes further and covers the follow-on question: there is no way to divorce your spouse and marry someone else, which is why only one set of marriage furniture can be obtained per save.

Neither the official 1.0 patch notes nor the studio's own 1.0 FAQ mentions divorce or remarriage at all. Two of the four sources say no explicitly, and the two official ones do not raise the subject.

Is divorce coming in a later update?

Unknown, and nobody has said either way.

The wiki notes that it is not currently known whether a divorce mechanic is planned. The studio has confirmed there will be content updates after 1.0, but has not listed divorce among them.

Treat this as a permanent decision for the save you are playing now, not as a temporary limitation.

What does this lock in for the rest of the save?

Three things follow from the same rule, and they are worth seeing together before you propose.

The three furniture pieces your spouse mails you after the wedding are the ones you keep. Proposal gifts are a different thing entirely — those you can collect from all twelve candidates before choosing.

When is the last moment you can still change your mind?

The proposal itself, not the wedding.

The 10 Heart scenes can be retriggered with the Engagement Ring right up until you propose, so you can see every candidate's event and take every proposal item without committing to anyone.

The moment you choose "propose" instead of "postpone" in that cutscene, the free look is over. What happens between the proposal and the wedding is a separate question, and one the sources do not settle.

What happens to everyone else you were dating?

They become friends rather than exes.

The official FAQ answers this directly: if you were dating multiple characters before getting married, they switch to "best friend" status afterwards.

So the game does not punish you for keeping your options open beforehand. It only closes the door once, and only on the marriage itself.