Sex's not a big deal?
I think you had missed something in S1E1-2.
It's ture....Sex's not a big deal for the growing up Brian who's a playboy but it's a big deal while he's still a teenage.....so.....he said he forgot the detail of his first time (with his gym's teacher).....but how, anyone could not forget the first time. Furthermore, Brian and Micky had tried to do "the thing" but Micky's Mom usually around both of them and come to Micky's room without knocking or asking while they're young, so.....I think it's quite difficult.