We're not talking about pants though, we're talking about a tuxedo. I know that girls tend to have looser definitions of what clothing is gender appropriate, but the rules for even their clothing are more restrictive in formal settings and tuxedos especially are still viewed as male attire. Plus, when we have a situation of a lesbian wanting to wear one and take her dress-wearing female date to the prom, the possibility that it is indeed cross dressing becomes much stronger than the possibility that I'm cross dressing because I'm wearing jeans right now rather than a skirt.

Plus, I agree with Issac. Dress codes exists for a reason. Make an exception in this case, and what else will you be forced to make an exception for? Men in skirts? People in causal attire? Tube tops? Where do you draw the line?