Responses
It's honestly just a minor translation mishap!
The phrase usually goes as"batting for the SAME team", a fun lil baseball themed euphemism for being a homosexual. Liking someone of the same gender. (Ex. A lesbian and a straight man are batting on the same team because they like women.)
Houjo is admitting to Kippei that he has liked a man before without putting a label (he might be fully gay or bisexual)
"Batting for the OTHER team" implies he's straight which he is most definitely not. whahahaha
I honestly love the reveal of Kippei's sexuality here. You start out thinking he has a girlfriend (his sister's words and him not saying anything to dispute them). And then he refers to his significan other as partner. And when he get the phone call you also can't tell the gender of the caller (well Eng translation. The OG Japanese might be different) and then you realize along with Honjou!!!!
And Honjou didn't offer the information that he himself is gay. How he handled the coming out with such grace and empathy especially when Kippei is upset. Arghhh honestly poetry.