Is the following okay:
GM: "Brother Jacob tries to persuade you to marry him to Sister Amelia."
PC: "Nope, I won't do that."
GM: "He's insistent - we have a conflict."
The stakes: if the GM character wins (and both are still alive!), the PC marries the NPCs.

I'm asking about a general thing - the above is just an example. Is it okay for the GM to initiate conflicts, the outcome of which might force players to take actions they don't want to do?

Yes, I think so.

It is no different from one of the players starting a conflict that will force one of the other players to do something the player doesn't agree with.

Of course it depends on the player to be able to play his character as the character would act, not now the player would want to act. Other wise it wont work out well.

I think there's a certain limit, which is hard to define really.

Also, I think that in general it's a good idea to let the players start the conflicts. The players (and Dogs) deciding whether to marry people is one of the most cool things in the game.

"What's at stake is: does she convince you to [whatever]" conflicts are fun. I fully endorse them.

-Vincent

Thanks, Vincent. That's great :)