Can 'The Beloved Farmer' achievement be obtained even if you're married?
Solution 1:
It is still possible to reach 10 hearts with all other Stardew Valley residents, which does not require giving them a bouquet. After marriage, your significant other will be able to have up to 13 of 12 hearts.
Oddly enough, you are still able to give bouquets to other marriageable candidates after marrying with no direct consequences that I've been able to observe.