Deano’s answer to: “Is it bad form to be the wedding date of a friend of the opposite sex if you are in a serious relationship with someone else?”

Nope.

Ignoring all the case-specifics that would "clear you" – being homosexual, for example[*], it really just comes down to intent… And so long as you both have clearly communicated intentions with each other, there's really no issue. Weddings, even very formal ones, tend not to be too big on 'the rules' these days.

So long as he/she was allowed a +1 on the invitation, then any escort should be acceptable, so long as the other terms are met (a no children clause would, of course, make bringing the 8 year old nephew or kid brother along a faux-pas to say the least).

How easy it is for either of you to explain the situation, or even how much you feel explanations are needed, is far too contextual to get into here, and basically has nothing to do with general wedding protocols.

[* Based on the question's wording, a homosexual with an "opposite sex" date would be generally regarded as harmless all around. ]

Is it bad form to be the wedding date of a friend of the opposite sex if you are in a serious relationship with someone else?