pro shipper in fandom terms usually refers to "problematic ships", incest, power dynamic issues, problematic age gaps (someone over 18 and someone under) etc. It's not they don't allow shippers, they don't want like, Barbara x Jean shippers to interact with them.
"pro-shippers" basically mean people who support incest, illegal age gaps and other ships which would be deemed problematic so they're not really welcomed by a lot of people. they also tend to use the argument that "it's just fiction" or "these characters aren't real" which would be true in some cases but they use it to justify shipping siblings or shipping a character with their parental figure lmao.
some youtube vids abt the "proshipper vs antis" discourse for more context