My opinion, since modern Vulcans suppress their emotions, the only logical reason to have a mate is procreation. So the vast majority of society is "straight" as a result of that logic.
At the same time, I would argue that it is an illogical activity to exclude someone of value to society based on who they have chosen to mate with. Logically, unless their choice of mate somehow affects their job/employment/task (whatever you want to call it), I don't see a logical reason why their choice in a mate would matter to anyone else.
The only logical argument I can find in all of this, is that choosing a mate based on feeling/preference, instead of logic, might demonstrate that an individual is more emotional and therefore less logical. And I think we all know how Vulcans feel about things that are not logical and/or things that act upon their feelings....
Personally, I don't see that having a preference in a mate, even one that steps outside the heteronormative, is a flaw in their logic. If you enjoy your time with your mate, and that makes you a better, more productive individual, then I fail to see a problem.