Alternatively, male sexual development might be more varied than is commonly believed.

Or the Malfoys might put a lock on their kids' early sexuality to make them easier to control. Something like that might make arranged marriages less prone to drama.

I think this might actually be feasible. Malfoys are probably not likely to have intimate enough conversations with children from other families that something odd is going on, and could write off what other children imply as just big talk.

Or the children could find out the truth relatively early, and buy into a family belief that this is the sensible way to do things rather than an imposition.