tralala: Sadly it may be depressing, but it is absolutely true. OP is not related to the groom or the bride, so therefore is not a shoe-in.
Don't call my view ignorant, it is not. It is looking at the actual facts of the situation, removing all emotion and saying it how it is.
I am not commenting at all on how much or little a step parent is or is not loved. Quite clearly the only real familial connection to the groom is via the parent they are now married to.
Simply, the parents of the bride and groom have roles, responsibilities that are traditional in a wedding, there is a traditional way of doing everything, venues also can dictate how plans are made. STEP-Parents don't have these roles, they are invited because of who they are married to now, not necessarily their relationship, nor even closeness to the bride or groom. In that respect, they are there to accompany their spouses, not to take part in the ceremony.
Of course if there IS a special relationship then it is natural that the step-parents are invited, but it is still an honour. It is lovely and testament to the step parents if a marrying couple wouldn't dream of not including their step parents, but not all family dynamics are alike.
At the very best of times, Seating plans are the 8th circle of hell.