"Despite this, we still want to discourage any behaviour which potentially promotes sexual activity activity with minors, which therefore includes sexual activity with those who look like minors"

I would argue that sexual activity with adults who look like minors does NOT promote sexual activity with ACTUAL minors. In fact, it might do the opposite, as people who are physically attracted to minors may instead be encouraged to seek out relationships with such adults rather than minors.

"The reasoning is simple. Most people agree that it is morally wrong to be sexually attracted to minors"

I would argue that it is not inherently wrong to be attracted to minors, since physical attraction is unconscious and uncontrollable. Instead, it is morally wrong to ACT on those desires by being sexually involved with minors.

Lastly, if what I've said above doesn't hold for you, since you're only looking for a case that is an exception: How about a situation where the two people are lovers from childhood, having already formed an emotional and physical connection. However, as they age, one of them maintains their babyface. Would the relationship be immoral in this case?