... If the friend meant you can't put the dick in an ass and then directly in a vagina, that's a legitimate concern, you can get infections from that. At least change the condom.

Otherwise... is the friend being bi/homophobic? Dating a girl doesn't make a bi man less bi. Gay men can also have safe sex, so I don't get how his sexuality has anything to do with this. If you have any concerns, you should talk to the man you're seeing. You deserve to know since your body is also involved.