can someone please explain this

my boyfriend & i we're making out, i was on him & my stepdad walked in, but when he talked to me later, he told me it's normal & that he doesn't care, but to be careful so my mother doesn't catch me (she's very overprotective). anyways, my boyfriend's really mad and told me that it was "really stupid" of us, and almost made it sound like he was blaming me for this whole thing. we didn't get in trouble & no one else knows, so why he is SO upset? i told him to text me when he's cooled down.