I agree...I just married her and all she does is thank me for resolving her problem. SUPER annoying.
Its actually a bug. Which I hope Beth fixes:
This is what I found on the wiki:
After completing her quest she will always greet the Dragonborn in circumspect fashion by hesitantly thanking them for Solving her "problem". However, this does not change once the two are married. This prevents the option to marry her.
This above bug only happens if you only kill her ex-lover and not the "sister". If you kill both, you can still marry her. The dialogue problem exists regardless.
PC users can fix this by using the console command "resetquest DB03", which removes the completion of her Dark Brotherhood quest. If still on the quest line for the Dark Brotherhood, this will bug Astrid and remove the ability to complete the quests she gives.
If she continues to thank the player after the above fixes have been attempted, try using the console command "player.setrelationshiprank 000198f8 4" By entering this code she likes the player instantly and the option to marry her will be available.