If you're playing the Bad Girl route, why did you choose David over Michael? He's the bad boy, and by not choosing him, you get David automatically. It's really simple, have sex with Michael during the party, and David doesn't get a look-in. He just becomes a guy she occasionally talks to at work.
Lucas, is all about the cheating. Victoria can't become a cheating whore, with nobody to cheat on. And Lucas is naive and stupid enough, for even a girl like Victoria ( who is hardly devious), to pull the wool over his eyes, with barely any effort. He's the perfect fall guy, who virtually nobody will feel too bad about being betrayed, since it's very difficult to like him. And he pretty much gives her an excuse to begin looking for other options, by kind of splitting up with her intially. Enough so that even Victoria, can begin seriously doubting whether they have a future. And of course, they don't have a future. Because if they did Lucas would feature in the relationship stats as well. The fact that he doesn't, shows that eventually they are going to split up. Once the Dev has had fun, getting Victoria to tease the fuck out of Lucas, and probably starting to enjoy cheating on him ( on the bad girl route at least).
That sex scene at his uncles, was important to demonstrate to Victoria, that Lucas was never very good in the bedroom. Back in her small home town, he felt like a big deal, because he was the son of a well-off local businessman. But in Big City, he's a fish out of water, whilst she's growing in confidence and totally out of his league. That sex scene brought everything into sharp focus for Victoria. Afterwards, she knows that Lucas can't satisfy her, the way other men in her life can. And she realises her feelings for him have changed, and she doesn't really love him anymore. But she feels guilty, because Lucas has given up everything to be with her, just like she asked him to. For a girl like Victoria, it's only right that she gives him a chance, even if she doesn't really want to. We know that ultimately she's probably going to dump him, but at that point she's not sure, and she's worried that Lucas will be so hurt, that he'll do something really stupid. So she makes, what is almost certainly the wrong decision for Lucas and her, but the right decision for the cheating part of the game, and decides to carry on being his girlfriend.
Later, we get the beach scene with Lucas, plus Harry and Miles. And that demonstrates where Lucas really fits in, in this game. With Victoria, teasing and messing around with both guys, right in front of the hapless Lucas, who is too naive and unobservant to realise what's really happening. One assumes this will eventually lead to Victoria having sex with other guys, with Lucas totally oblivious that his girlfriend has become a total slut, and gets off doing it with him in close proximity. So I think that sex scene with Lucas, at his Uncle's apartment, played a very important role, for what was to come.