The way I see it, just because they're a certain type of person on one route, doesn't make them the same kind of person if they make different choices. The choices you make define who you are as a person, and you need to try to see these people as completely different individuals, on the different routes.
The problem with Cindy, is that Ian has to betray one of his best friends, to be with her at all. Even if he only has sex with her, and nobody else. He's still doing a bad thing, and expecting Cindy to become a cheating slut, so they can be together. In a sense, it's him and not Axel who has opened Cindy up to corruption. So it's pretty hypocritical of him to complain, about her so far, relatively innocuous flirting with Axel, when what they've done together is infinitely worse.
If Ian and Cindy are dating, then so far nothing has really happened between Cindy and Axel, and maybe nothing ever will, as long as Ian doesn't allow his insecurity and it's resulting possessiveness, to anger Cindy and drive her into Axel's arms. I think if Ian chooses to be cool, and supports Cindy's ambition to be a model, and trusts she won't succumb to Axel's charms, there is a high probability nothing will happen.
At the end of the day, Wade did the opposite, and he lost her.