Dude, leave it alone. That's going to be an ackward discussion with your mum, she probably already knows or suspects, and quite frankly it's not the end of the world. Why the hell would it even matter to your mum? Rather than pestering her for sex, he's out letting it like hundreds of other guys around the world.
Unless you know that your mum will actually care about this, like if she has some sort of special feeling about this, just leave it alone, your only going to stress relationships(if not your mum and your step-dad, then your step-dad and you). Oh, unless you don't like him, then by golly, GO FOR IT!!!
Unless you know that your mum will actually care about this, like if she has some sort of special feeling about this, just leave it alone, your only going to stress relationships(if not your mum and your step-dad, then your step-dad and you). Oh, unless you don't like him, then by golly, GO FOR IT!!!