Since you never had a girlfriend, Mom wondered if you might be into guys. If you weren't sure, she had a simple way to test it using a cock dildo. She'd have you watch her lick it and suck it, then she'd have you try it yourself. Once she saw you getting into it, she was pretty confident you were into cock. However, if that was the case, then you needed to learn more about gay sex, which meant the dildo would have to go into your ass.
She'd instruct you to get comfortable, to stroke your cock, and she'd gently penetrate your asshole with it. She'd go harder and faster, hitting your prostate, while encouraging you to cum. Once she saw your big load, she determined you were probably gay. If you found yourself thinking of a woman, however, you might be bisexual. All she knew for sure was that you loved cock.