I don't want my girlfriend to play soccer
Why don't you want her to?
I think she might meet other guys and date them instead.
You have no right to tell your partner if they can or cannot play soccer with other people. She has the right to play in co-ed sports. Have you thought about talking to your girlfriend about this?
It's not like I'm going to hurt her or something.
You need to respect she can have her own friends. If she is dating you she isn't worrying about dating other people. Have you considered joining the co-ed team?