I wouldn't go. Personally, I would just feel a little uncomfortable knowing that the host's first preference would be for me not to be present.
I don't think that's entirely fair. The host's first preference wasn't for the OP not to attend, there just wasn't room enough for the guestlist to extend to her. It wasn't a preference for the OP not to attend but a higher preference for somebody else to attend instead.
I think it was though, because if the party had gone exactly as the host - the husband, not the GoH - wanted, OP would not have been invited.
I realize this, but I still don't agree that the preference was for the OP not to attend. It's a semantic difference I'm not sure I can explain, but I'll try.
Of course this is based on assumptions as I can only guess at Bob's thought process.
"Prefer the OP doesn't attend" - this is a negative statement where Bob has considered it and consciously thought "No, I'd rather that she isn't there."
"Prefer somebody else attend" - this is a neutral statement to the OP, where Bob may not even have considered the OP personally, but thought "I'd like for these people to come, and unfortunately that means we're out of room and can't invite anybody else."
In the first case the OP was specifically not wanted. In the second case, she just didn't make the A-list.
Does that make any kind of sense at all?