Does having a ruin a serious relationship? My bf said he didnt care if I had a gf on the side...?

This morning my bf said he wouldnt care if I had a gf on the side when asked about it. I was just asking to be stupid, was half serious about it as Ive never been w/ another woman other then kissing while drunk (alot of girls have done this before) & I was surprised about his response. Do you think it would ruin a serious committed relationship to have a or call his bluff & possibly explore a friendship or more w/ a bi girl??