There actually are good reasons for the policy of not allowing mixed-sex room sharing, although not all of these would be reasons in a perfect world.

Some reasons boil down to the fact that women tend to be less strong than men, and so can reasonably fear physical attack by men. Some boil down to youths being emotionally unstable. The first reason is that if, for whatever reason, people are being allocated partners, rather than choosing them, and a mixed-sex allocation was made, there is the danger of the woman being attacked. You may object that universities should just avoid mixed-sex allocations, but if you do, you might go on and suggest that they institute a policy of not making mistakes. It doesn't help in the real world.

The second reason is that universities may reasonably decide that no (wo)man can make an informed decision about the advisability of living with a man. This is not necessarily true, but there are two cases in this one point. The first is that the university might get sued if something bad happens (See above). Universities are particularly averse to this. The second part is that most couples are heterosexual. Typically neither partner is right in deciding that living together is a good thing when in university, as a) They haven't (normally) been together that long, and b) If they do break up things get REALLY messy when living in one room. This reason is the big one, as it actually happens very commonly, while violence is not that common in reality. Basically, there is no way of preventing homosexuals from doing this, but that's just life.

Of course, many universities will allow mixed-sexes sharing the flats that they provide. This is typically not a problem, however, as they don't usually allow people to pick their flats.