No it actually kinda common rule in mmo because it is a public guild. Some common cases include
1. Some player only act kind / friendly to certain players and not others so when team up, it can be awkward to the guild members that isnt part of the "friend group". This may lead members to be inactive in the guild or make the guild seems exclusive despite it being public guild
2. Personal relationships may lead to fight for example if 2 player is dating and having a fight. Other members may take side and thus your guild environment is definitely gonna be toxic which may lead to guild dismissal
3. To protect members. Its a public guild after all you dont know if everyone there is an actual good person or not. There also usually minor in this public guild and they are more naive.
ARUGHHHHH THEY'RE SO CUTE!! also what's with "no personal relationship" rule lol what kinda bitchass friendless loser made that rule sounds like someone who can't find a friend for themselves so they decided to make that rule so everyone doesn't have a friend in that guild