well i will not went though this torture but they just need to become a cold "couple", you need to focus on the cold part this way the grand parents will see how stupid this idea was, then they could have a open relationship where they see other people oh well i guess that the granfather didn't say anything about loving each other oh no i guess that just marrying is enough how sad. This would be the most realistic aproach but since this is a manga of course they will eventually love each other after 100 chapters