probably that he's going to see a cute girl next door, and start a conversation with her, and invite her into his house, and they'll have consensual sex. Maybe the details vary slightly; perhaps she's a niece or something, or maybe she's a girl they meet online. Either way, they go someplace where they can be alone, and have sex.
And in the proposed future society where sex with a consenting kid is legal, theoretically this would not get him in trouble.
What could go wrong with that scenario, in practice? Well, a lot of stuff. She could accuse him of rape later, since what they did happened behind closed doors and there's no proof of her consent. Maybe he's a rich guy like Michael Jackson, and she wants to shake him down for money. Or maybe, like Brett Kavanaugh, he's going to be nominated to the Supreme Court, and she'll come out of the woodwork decades later to try to sabotage his confirmation. Or maybe, like Paul Nungesser, he'll get accused because of her mental illness that drives her to become a cause célèbre by carrying a mattress around.
Men who have never been accused of rape probably assume it wouldn't happen to them, because they only hang out with quality women, and they behave themselves well, and there's lot of evidence of consent such as love letters and amorous texts and whatnot. That doesn't necessarily prove that every single time they had sex, it was consensual. Even if the relationship went on for a long time, she could say later that it was an abusive relationship that she hung onto long after it would've been good to have dumped him, because she was young and naive, or he was being controlling and threatening her, etc.
We see this problem even when the relationship is between two adults; you can imagine how it will be if men are allowed to have consensual sex with young girls. The girl will be able to say that he was stronger and overpowered her, and this will sound credible, given their difference in size and strength. In some cases, she may say that because of her age and naivete, she didn't give truly fully informed consent. Maybe to a jury, a little girl could be a pretty sympathetic witness, when she's accusing of rape an older man who, according to the evidence, had sex with her.
So there could be a big problem with false rape accusations arising. Even if the girl was documented to have said she was coming over for consensual sex, that doesn't prove that she still consented at the moment they had sex. And even if she took some sort of test to prove she understands, academically, what sex is and what its implications are, some might argue that she still does not have the life experience to fully grasp what is involved in all that, or that she had the gumption to be able to resist the advances of an older man if she wanted to (there are some young people who are capable of asserting themselves in some situations, but a lot of times, young people have a tendency to passively go with the flow instead of pushing back when they find stuff objectionable).
If you believe that sex with girls should only be allowed with the girl's consent, I would ask, what are you going to do about false rape accusations; how will you prevent those from ruining men's lives? There could actually be more men getting accused of child rape, once consensual sex with kids is legal, than there is now, since more men would be trying to have sex with kids, or running onto kids in places like dating sites, and getting into compromising situations.
You might respond, "The accused will simply be considered innocent until proven guilty beyond a reasonable doubt." In that case, how do you propose, in cases where the girl really was raped, that the prosecution is going to meet its burden of proof that she did not consent to sex? Right now, all that's required is that they prove that the accused had sex with her, so it's easy to convict him, whether she consented or not, since her unwillingness is not an element of the offense.
In other words, how would you prevent men from getting away with raping young girls, in a society where consensual sex with those girls is legal? What would keep it from becoming like the situation with adult women, where a large percentage say that they were raped and didn't report it; might it not drive down the rates of reporting and prosecution of child rape, if the unwillingness of the child were made an element of the offense that needed to be proved, and thus it were harder for such a prosecution to be successful?
If there is going to be a rise in the rate of child rape following such a reform, what is your argument for how the benefits of legalizing sex with children outweigh that which, being opposed to rape, you perceive to be a cost?