Hi❤ thank you for leaving this comment because I'll hopefully be able to clarify some things to you.
While the arranged marriage isn't avoidable, in this setting is very common as a practice, especially for monarchs and nobles, this game will heavily drift away from the more uncomfortable sides of it.
There won't be any expectation of a children or pressures by the court because Arthur has already named his heir. Since MC is gender selectable, it is also taken into consideration that the marriage was never for the purpose of producing an heir.
On the topic of MC, they have the freedom to have a purely platonic relationship with Arthur. Should they choose to pursue other ROs, or no one at all, Arthur will be helpful and supportive either way. Basically, the bond that MC and Arthur have is set and will always be a strong and central point in the story, but not linked to Arthur's romance route!